Today's digital camera sensors pick up everything, so be sure to take care of all hair removal prior to a shoot, but leave at least 2-3 days between when you wax/shave any sensitive areas (such as your bikini line) and your shoot date. This time is necessary to allow for any irritation to disappear before your shoot.
Take care of your nails. Be sure that your nails are neat and well maintained. Short nails are preferred. A natural, non-colored, or French manicure look is preferable. If you plan on posing barefoot, be sure your feet and toes are in good condition.
If you plan on showing your smile, be sure that your teeth are ready for prime-time. If your teeth are discolored, you may want to consider having them cleaned/whitened.
Eat healthy. Drink plenty of water. Avoid fatty, greasy, and salty foods prior to shooting. Please avoid alcohol or anything that will make you puffy or bloated. Also avoid anything that may stain your teeth.
Avoid sunbathing at least from 48hrs prior to your shoot.  There is nothing more unflattering than red, sunburnt skin.
Avoid wearing tight-fitting bras and panties on your way to the shoot.  They leave unglorious red marks on your skin, and these are easily picked up by the camera especially if you intend to pose in a bikini etc...
It is also important that you get a good night's sleep. You will not only feel better, but you will look better if you are well rested and energetic on the day of your shoot. Dark circles and bags under one's eyes are not attractive.
Please eat a light meal before you leave. Food is fuel and if you skip your meals before shooting, chances are you'll run out of energy, lose focus, or become irritable. None of which will improve your photos.
Please bring bottled water, snacks, and anything else necessary to keep you going, but avoid bringing anything that will stain your teeth. 
To avoid squinting due to bright studio lights or sunlight, please do not wear sunglasses prior to your shoot.
Beware of “new” clothes since you may appear uncomfortable.
You don't want your clothes to take emphasis "away" from you.
Avoid loud patterns such as checkerboards, polka-dots and stripes.
Go with garments that are medium to dark in tone and are solid in color.
Put your clothes on hangers and iron/steam the night before, and check for defects.
Bring several pairs of shoes of several types-although they may not be seen, they complete the outfit and give a sense of the character.
Ladies, don't go overboard on the makeup. If you choose to do your own make-up, do it as if you were going out to dinner with a friend you haven't seen in a while. You want to look great...just don't over do it! Simple, clean, and fresh is best...emphasize eyes, lips, contour (face shape).
Do NOT use a shiny base layer (foundation), or any other powder that has a glimmering or sparkling effect.  This causes skin shine as soon as the flash fires, and is almost near to impossible to remove in post production.  Always use a matte foundation.
Bring your make-up kit with you so you can make adjustments, touchups as necessary.
For guys...a light powder foundation can reduce imperfections and an oily appearance.   If you are concerned about looking made-up, then at a minimum you will need to bring an eyeliner and face powder with you to your shoot. Face powder helps to subdue face shine from oils in the skin. 
Don't hold your breath for a pose. The concentration usually shows in the picture.

Hold in your stomach to give a more toned appearance to the abdomen (even if you are in great shape).

Most people, including models, do not have very good posture. Unless you're going for a casual look, keep your back straight and your shoulders up.

Don't keep both arms entirely straight unless directed to do so for a specific pose. Bend one or both arms, even if only a little, to make the pose look less artificial. Likewise, don't keep both legs entirely straight.

Don't always look straight at the camera. Instead, use a variety of head and eye positions: Try turning your head, tilting your neck to one side or the other, or looking off to the side for some poses.

Don't use a big smile for every pose. Sometimes try a small smile, a pout, a laugh, a scowl, or even a frown, to give some variety to your facial expressions.


Listen to what the photographer tells you.

He can see you through the camera. You can't.

Additional Posing Tips for Plus-Size Models:
Rest most of your weight on one foot or the other instead of evenly on both feet. Doing so makes poses look less artificial and gives a slight slimming effect in most positions.
When sitting or reclining, roll one hip up slightly from the resting surface so that most of your weight is on the back of one thigh or the other instead of evenly on both thighs. This gives a slight slimming effect and makes the pose look less artificial.
Avoid having your arms hanging flat against your sides. Instead, bend your arms slightly, place your hands on your hips, or try other poses that involve alternate placements of the arms.
Keep your chin up to avoid chin wrinkles or double chins.
Posing Tips For Emphasizing Cleavage:
Lean forward.
Bring arms together at the waist.
Clasp hands together below the waist, keeping arms straight at the elbows.
Cross arms.
Posing Tips for Minimizing Cleavage:
Lean backward.
Keep arms apart.
Raise arms above shoulders or above head.