I always tell people who are taking photos with someone for the first time to please travel with a friend!
Before going with a new photographer, do the research. Check out all sites related to the photographer and if they have a networking site with credited or tagged models go talk to the models to find out how professional the photographer is.
Bring a change of clothing, fresh socks, traveling shoes, towel, robe, nude panties and an interchangeable bra. Always be prepared for locations and weather.
Cellphone: Make sure your cell phone battery is fully charged for emergencies and let a family member know where you are going to be AND when to expect you at home. If you are running late, give someone a text to inform them where you are at that time.
Never do anything you are not comfortable doing. If the photographer tells you to do something you do not want to - tell them NO and if they are still unwilling to take NO for an answer...leave!