Know hygiene, know what sizes mean what, know what it should feel like, and what it shouldn't. Know what area's of the body should be avoided (not because it scares you but because real damage could occur). Know what to about bleeding, bruising, or any after effects from the play. Know your body, and any physical or medical limitations. The medical one is important as your blood pressure or sugar level may need monitoring as you go through the experience. Make him aware of your fear, and help him understand why you have it. If it's not based on a past experience he has more latitude to play with your head than if you have had a traumatic experience related to the activity.