what I do before and during a tattoo is this,
1)don't party hard the night before(no alcohol),get a good night's sleep, & pre-hydrate the day & night before with water or something with electrolytes(gatorade & most sports drinks)
2) eat a good meal right before
3)bring 2 or 3 bottles of water or sports drinks with electrolytes,a banana,skittles & M&M's,and maybe some jerkey all to munch on & drink during.(keeping your sugars,proteins & potassium levels up).
4)*like was said,if you have high anxiety,maybe take something like 1 or 2 mg of Lorazepam to relax you a little before & during the tattoo.
5) most important ! NEVER DRINK ALCOHOL BEFORE OR DURING A TATTOO !