before your appointment

Eat a large meal before you arrive.
Drink lots of water.
Avoid alcohol the day before and day of.
Don't take any painkillers or illicit drugs. They can thin the blood and thinner blood means more blood which can lead to a more difficult tattoo.
Bring some snacks, a non-alcoholic drink, a phone charger, and cash for your tip.
Please, wear deodorant and don't douse yourself in perfume or cologne. 
You will want to dress comfortably and in dark colors to avoid ink/blood stains.
It helps to stretch beforehand since you'll be in the same position for a while.
Stay calm and do breathing exercises or whatever it takes to get you to a zen-like state.
Set aside more time than you think you'll need, you don't want to rush this.