Know your Assets and Liabilities
When you work with a photographer, it is useful for them to know what you see as your strengths and weaknesses. Sensitive that your stomach is not as flat as you'd like? Tell the photographer. Bruise on your arm leg from playing soccer? Tell the photographer. Moles or birthmarks? Tell the photographer. It is easier for the photographer to work around something if they know about it, and it will ensure that you get images that you are happy with.
Be a Ham
The difference between good photos and those that catch the viewer's attention is engagement with the person in the image: you. Be someone engaging. Make your expressions over-the-top; it is easier to scale them back than to scale them up. Think about the people who most stand out; they tend to be characters with seemingly out-sized expressions. Photography is theatre, but you only get one image to tell the story and you have to use your facial expressions and pose to tell it. Make it count.
A tip: Practice your expressions in the mirror when at home so that when you get to the studio then you know what they look like.
Any time one body part meets another, avoid pressing them together as much as possible. Chin on hand pose? Lightly place the chin on the hand. Arms crossed? Place the arms lightly over each other.
A tip: Imagine your skin is fragile like a butterfly. Every pose involving contact should be light and delicate, ready to move to the next pose.
Don't Point your Joints at the Camera
Specifically, avoid pointing elbows, knees, fingers, and armpits directly at the camera. Elbows and knees in particular are where skin naturally bunches so are everyone's least flattering asset. Keep them pointed way from the camera by, for example, sitting with knees pointed to the side, bringing the arm up to frame your face from the side, or turning the body slightly so that the armpit isn't pointed directly at the camera.
If it Bends, Bend it
This may appear to contradict the some of the Rules, but there is a happy medium. Creating joint angles makes the image more dynamic. Even a slight bend is enough, just avoid locking your joints.
The general rule is that if you have a joint visible, bend it. This makes it clear where the joint is and both makes the image dynamic as well as gives a sense of proportion.
Avoid holding your breath during the shot. It will tense the muscles and you won't look relaxed. Practice breathing through the shoot. One way is to breath in before the shot and slowly exhale through your mouth or nose through the end of the count-down and past the shot. With practice this will become second-nature, and you will feel more relaxed (thus, produce more relaxed images).
Try to avoid doing the same thing on both sides of the body. You will already be starting to do this if you are tilting the shoulders and hips. If one arm is down, try the other up. If one leg is straight, try bending the other one.