First and foremost, arrive on time.
Make sure you have a good planner and keep it on you. Open studios might call you when you're least expecting it.
Have business cards, and have at least two handy at each modeling session you work. Networking is the name of this game.
Have a robe with you, so you can easily change. It's good to have two at minimum.
Try to stretch before the session starts.
Exercise as much as you can at home. Holding beautiful poses sometimes include raising your arms for an extended amount of time.
Be a muse. Pretend you are a graceful ballet dancer, a strong athlete, or whatever comes to mind to help you model. You are there to inspire.