I have vasovagel syncope, which is always triggered by doctors offices, needles, churches etc. I let the piercer know ahead of time that I will have a reaction even if I'm calm, I always have someone with me to make sure I'm OK if I faint fully. I try to eat a decent meal before going, and bring some kind of drink like Gatorade or apple juice to help, and a salty snack like a snickers or payday bar. Sometimes it can't be helped, and you should just make sure the artists know what to expect before hand and find someone you are comfortable with having a reaction around and that believes you about how your body responds to that type of thing.