Good prep advice already ....
I will add a BIG second to practicing inthe mirro. Learn your "good side" and flattering poses. I would do all kinds of things - standing, stretching, silly, glam, action (like mid-jump), working out (lifting light dumbbells), sitting, etc. You want to be prepared for ANYTHING. Look through the magazines and see what pictures are in there. Mimic those in your room so you can practice.
Also, do NOT hold back in the shoot. Don't feel silly - pose your little heart out. Most fitness photog's I have talked to WANT a model they do not have to "pose" themselves. They LOVE model's who just "do their thing" while they snap pics. Be natural, act natural.
Bring some FUN music - that helps so much. You'll be be-bopping and dnacing around - plus, it makes you feel so upbeat and confident.