How do I Find a Photographer?
Every model needs a portfolio and composite cards. These are the basic marketing tools of the model, just like a resume would be to any other job seeker. At a minimum, a model should have a good clean head shot, and one good, simple full-length shot (in something that will show off the shape of the model -- a fitted dress, a leotard, swim wear, lingerie, etc).
If you are already with a reputable agency, they will likely have a list of competent photographers who you can choose from. Also, other models might be able to give you suggestions on the photographers that they have preferred to work with.
Unless you are posing for the photographer's work in exchange for free portfolio prints, you'll also have to consider a budget. For a head shot and full length shot, figure anywhere from $50 - $500  for the session and an 8x10" print of each. If you are getting many more looks (for a composite card), expect to pay anywhere from $250 - $1500 for session and prints.
Before having photos done, it's best to talk with your agency to find out what their suggestions are. Again, they can't force you to have certain types of photos taken, but if they are a legitimate agency, they will want to give you advice on how to make you most marketable (again, the more money you can make, the more money they can make). Some agencies have very specific suggestions such as number of shots taken in studio and number taken on location, where locations shots are to be taken and what type of backgrounds, what type of studio background and what type of colors, print sizes, pose length (close head shot, medium length to waist, 3/4 length to hips, full length), film, and even lighting. It's best to discuss these requirements well in advance so that you can let the photographer know (and have time to play) the shot you need. For your portfolio work, you should plan on paying for a stylist/make-up artist. You need to look you very best, and paying for a stylist is like getting a free course in "how to make YOU look beautiful." Again, your agency or photographer should be able to recommend a competent professional in an affordable price range.
Remember that as a model, you are responsible for your own decisions and conduct. You need to let photographers know if you feel that their conduct or requests are unprofessional (or making you feel uncomfortable). If the behavior continues, you have the right AND responsibility to leave a photo session immediately. 

How Should I Prepare for a Photo Session?
Modeling is perhaps the most difficult of professions, because you are judged on both your performance AND your looks. There are many simple things you can do which will help you appear professional and successful (and therefore leading to more jobs and requests).
Weight is ALWAYS an issue with models. While you should not be anorexic, you need to maintain desired weight levels and exercise on a daily basis to keep muscles toned.
Along with exercise goes proper healthy eating habits. This includes lots of fruits and vegetables as well as laying off the fats and snacks. Also avoid caffeine, smoking, alcohol, and other substances which will deplete your energy and health.
Don't change your looks before a job! Avoid last minute perms, colorings, or haircuts unless you are specifically instructed to do so. Longer hair is almost always preferred on models because it is more versatile (you can make long hair look short, but you can't make short hair look long).
Avoid wearing make-up except for actual jobs and interviews. Make-up of any kind blocks up pores in the skin leading to unhealthy skin (and sometimes acne and pimples). Bath a couple times a day and use appropriate skin care products to keep your skin as healthy as possible.
Avoid the sun! Getting sunburned is not only bad for your career but it's also unhealthy. Also, tan lines are nearly impossible to cover up with make-up. It's much easier to get a pale looking model to look tan than it is to get a tan looking model to look pale. If feel the need to look "tan," use high quality self-tanning products which "color" your skin rather than tan it. The key is to apply it evenly (often you need help with this) and you need to stand around nude until the product is completely dry.
All models need professional manicures and pedicures to keep their nails in great shape. Be CAREFUL! Bruises, scars, scratches (and worse yet, tattoos and piercing) wreck your product (your body). Always remember that you body is your "product" and keep it in as pristine shape as possible.
Women (and men depending on the project) should shave or wax unwanted body hair a day before the job. This will give enough time to allow any red marks to fade. When shaving, always shave in the same direction that the hair is laying down. If you shave "against the grain" you'll end up cutting the hair off below the surface of the skins, and you'll have "razor burn" as the ingrown hair tries to force a new hole through the skin for itself.
Always arrive at each job freshly bathed and showered. Hair should not only be shampooed but also conditioned to give it more body and "height.". Bring your own curling irons, hair spray, combs, berets, brushes, etc. that you'll need for your hair.
For most clothing shots, the model should expect to pose without underwear on. Because underwear shows up or creates lines or bumps in the fashions being worn, typically the model will wear ONLY the clothes provided. If you will be posing in revealing clothing such as lingerie, swim wear, or nude, come to the photo session wearing loose fitting clothing and no socks or underwear. Anything that presses on the body creates reddish "pressure marks" which take up to an hour to go away. If you have a shoot like this, do not wear any tight clothing or clothing with elastic in it for roughly 12 hours before the shoot.
If you are "Testing" or doing "Portfolio" work... Practice walking, posing, and depicting different facial expressions/moods on a regular basis. Pick out clothing ahead of time which will give you a variety of looks and which flatter your figure. Avoid distracting patterns which lead the viewer's eye away from you. All black or all white outfits are always good choices. Similarly, clothing with simple and classic "lines" also are beneficial. It's best to bring a much wider selection of clothes than what you will need so that the photographers and have choices to pick out what they feel will work best.

What Do I Need to Know About Makeup?
You should learn as much as you can about makeup and makeup products. Makeup is designed to enhance positive features, minimize negative features, and draw attention to specific features. On many shoots, there will be no make-up artist, and you will be responsible for applying your own make-up. Even on shoots where there is a make-up artist, you may need to supply your own make-up, and you should learn the best colors and products for your particular skin type.
One important lesson you will find is that you "get what you pay for" when it comes to make-up. For photo shoots, you will not want to use any "street brand" type of make-up, but rather you will want to use professional "stage and screen" or "photography" make-up products. These cosmetics are much thicker than traditional make-up so that in one application they will smooth out the skin and fill in the pores. Often these are referred to as "grease paints" because that is what they were called in theater usage.
It is in your very best interest to spend some money on a professional consultation from a make-up artist who does stage and screen work (the persons at most retail cosmetic counters have little training and often give advice which is damaging rather than helpful).
One of the keys you will learn is to use "light colored" base/foundation. It looks very odd in photos when the face is several shades darker than the rest of the model's skin. Also, make-up has to be blended so that there are no "seams" or "ledges" where the make-up abruptly stops. Many times (for lingerie, swim wear, and nude photos) the entire body is covered with make-up. This is very time consuming and must be done carefully so as to appear natural. Another lesson you will quickly learn is about "contouring" or sculpting. On a flat photograph, you can "trick" the viewer by being clever in makeup application. Our eyes think that areas which are "lighter/brighter" are closer to us, and areas which are "darker/dimmer" are farther away. What contouring does is to use darker make-up on areas which you want to appear smaller (or farther away) and to use lighter make-up on areas that you want to appear larger (or closer). If a nose is too broad, you use a lighter color running right down the bridge of the nose, and use darker shades on both sides. In a photograph, this makes the nose appear "slimmer" than it naturally would. If you don't have good cheekbones, you can literally "draw them on" with make-up. Again, a light color is placed on the cheek where the top of the cheekbone would be, and a darker color is placed down the rest of the cheek. This makes the cheekbone "pop out" of the skin, even if there is not cheekbone at all!
One thing to be cautious of is the types of make-up you use. Since photographic film picks up ultraviolet light as well as visible light, your make-up may appear completely different colors once photographed. Most professional stage and screen make-up is formulated so that "what you see is what you get." The more you learn about make-up the better. Make-up is the one area of your appearance which is easy and inexpensive to change, and still highly effective when done correctly. Purchase several books on professional make-up techniques as well as request catalogs from a variety of suppliers of professional stage and screen make-up.

How Do I Learn to Pose?
The best way to learn to pose is to "just do it!" Get as much time in front of the mirror and the camera as possible. Clip out magazine ads and put together a "pose book" of 30-50 different poses. Practice these and find out which ones work best for you. A professional model should be able to easily move between many different poses which are especially effective for him/her.
Another good technique is to study your proofs and contact sheets from photo sessions. Find out what types of poses make you look the best "on film." Yes, what shows on film and what shows "in the mirror" of sometimes different. One key to posing is to "break symmetry." Poses look more interesting if arms and legs are "uneven" (or don't match left-to-right). A lot of times you'll see that a model has his/her body "twisted." This is very effective at making the model look like they are "in motion" because the photo looks like something has just caught their attention and they are turning to see what is was.
In the same way that you should "break symmetry," you should also practice your balance and condition your legs. Many times you'll need to hold a difficult pose which makes you feel "off balance." While it may feel terribly uncomfortable, it looks GREAT on film. Once you don't have to worry about losing your balance, you can have more fun with the photos, and your smiles will be much more natural.
Finally, one of the areas that models always seem to neglect are facial expressions. For any kind of commercial work, facial expressions (besides the "smile" which everyone is used to doing) are a key to getting jobs and to being effective in communicating emotions. Practice in the mirror for ten minutes every day making funny faces. Really exercise your facial muscles. The more conditioned those muscles are, the easier it will be for you to come up with creative gestures and expressions.
If you are careful in studying what works well for you, and you've come up with a set of poses and facial expressions that work great, photographers will love you. They will soon be passing the word around to "hire her -- she never has a bad picture."

How Do I Know How to Pose?
There are two keys to posing. First is to "break symmetry." The second is to keep "in the light." A person standing with arms at their sides looking right into the camera looks posed and "boring." However, look through the fashion magazines and not how the model's body is often twisted, with the hips pointing a different direction than the chest, and the face may be pointing yet an entirely different direction. Also look at the hands and feet. Notice how one hand is higher than the other and that the feet are often pointed different directions or standing on different "levels?"
It is this "broken symmetry" which makes photos look interesting and dynamic. It looks like the model is actually moving and doing something even though they may be perfectly still in the pose.
The other main rule is to "find your light." The photographer will normally have one light which is far brighter than the others. For the most part -- you want to point your chest and face toward that light -- so that the important parts of your body are lit well. Of course the photographer may tell you to point your body different ways -- but this is to emphasize different parts of the body. However if the photographer isn't giving you much for directions -- try to keep pointed "toward the light."
How you pose will depend upon the way that the photographer likes to work. Some photographers prefer models who "move to the music" and simply takes "snapshots" when he/she sees something they like. Other photographer's will meticulously pose every little detail -- down to what direction your fingernails are pointing. With the first type of photographer -- the best thing is to "KEEP MOVING." With the second type of photographer -- the best thing is to "NOT MOVE AT ALL." You will have to learn to adapt to the style of the photographer you are working with -- and he/she may have to adapt to you somewhat as well.
It's best to work with some really good and experienced photographers when you are just starting out. They will teach you some tricks and some methods which will help you look your best -- even if you are working with more amateur photographers in the future. If you start out working with amateur photographers who don't know what they are doing (or how to instruct you how to pose to make you look good) then you are just wasting your time. If on the other hand, you've learned some basic posing techniques -- then your time spent with ANY photographer will be more productive.

Any Other Advice?
Photographs can be your best allies in finding jobs as a model. You should be willing to "bend over backwards" and "work for free" on photographers pet projects whenever you can to get more experience, better portfolio photos, and better working relationships. You want to be every photographer's "favorite model" -- because you'll be the first one called when the photographer is casting a part you're appropriate for.
Many models make the dreaded mistake of thinking after they've had a few paying jobs that they should be paid every time they pose/model. This is the worst possible attitude a model can have. It's like a store that opens up with lots of advertising, and once it gets its first couple of customers -- figures it never has to advertise again. When you're doing "tests" and working "on trade" with photographers, you are building up your advertising, and you are doing "marketing" to keep yourself in front of the eyes of not only the photographers -- but all the clients of the photographer who might see your image.
Many times the client will ask the photographer to use "the same model as your poster." They like your look. You don't even have to interview for the job! You can't BUY better advertising than that!
In the same way, you should bring creative ideas to photographers for projects that you are interested in doing for yourself. Some photographers love a "challenge" to do something wild and new. The best of all worlds is when the two of you work together to come up with a piece to be published to promote both your skills (like a postcard which both of you can use in mailings, or a poster, or even a calendar). While most of the time you'll both have to invest your money in the advertising, if the image is good enough, it might even sell to a publisher -- netting the both of you a small sum and great publicity for your efforts.
No matter who you are or how long you've modeled, you should always jump at the opportunity to "do another job." If nothing else, it keeps you in practice. At the very best, the small and low-budget jobs are the creative works that win you the national contracts. Always work hard and never give up