Chaperones
When going to work for a photographer you have never met or in any situation where you do not feel in complete control it is perfectly normal for you to want to bring a friend along for moral support or for safety. This is expected and no genuine photographer would ever refuse permission to bring such a chaperone or try to dissuade you from doing so. in fact if they do then I would suggest that youdon't go on that shoot and find yourself a different photographer as soon as possible.
Some chaperones are however better than others, both for yourself and for the photographer. Make sure you bring along someone supportive of your modelling, rather than someone who is suspicious of everything. Some models with whom I have worked have brought along parents or boyfriends as caperones when these people do not wholly approve of modelling as something their daughter / girlfriend should be doing. In these cases the shoot is often strained and once or twice I have had to abandon a shoot because the chaperone is making it impossible to continue. The best chaperone is a close friend who knows you model and is not there to judge but to be supportive.
Release Forms
On almost any shoot instigated by the photographer (that is not one that you yourself have commissioned) you will be asked to sign a release form. The form of these varies from photographer to photographer but in essence they grant him or her the right to use the photographs however they wish in future without asking any further permission from the model. Being asked to sign a release is perfectly normal.
Tight Clothing or Underwear
If you are doing a nude shoot and you are wearing tight clothing - including underwear - when you arrive this may mark the skin and delay the start of the shoot by as much as an hour and a half in some cases. If you are particularly susceptible to skin marks you would be advised to remove anything tight before you arrive at the shoot so that no time - and therefore money - is wasted.