Well, I dont have a tat yet (because my parents are saying no, and I have no cash :P ), but I've done a ton of reading about it.
Eat a meal about 2 hours before the tat. Also, dont have
any booze or drugs (including medication), for 2 hours before the tat.
Reach the parlor early, because it puts the artist in a good mood, and hey, if you're as paranoid as me about health, you can watch the artist clean his equipment.
Do not bring an entourage. One friend is OK, but even that friend may not be allowed to watch the tat process.
Make sure you've got the cash for the tip etc.
Also, wear clothing that makes it easy to reach the tat area.
FInally, listen to what the artist tells you about aftercare.
Hope I helped