mutliple sessions and tipping

My next tattoo will be a 3 session piece. How do I tip? at each session or at the end?

i am 3/4 done on a four session piece and have tipped after each session. i am really happy with the work after each session so i try to let the artist know by tipping as much as i can afford. i also think its the little things too, not just money. sending them an email afterwards explaining how much you like the progress and enjoy working with them, bringing in snacks / drinks, or bringing in a cd if you talked about the band the last time. money is nice, but its the little things that people enjoy and will remember.

Sleewell I agree 100%...An email or a card saying they really love the work means more to me than any tip...or when a new client comes in and says that their friend ( brother,sister dad, etc etc etc) wouldn't let them come to anyone else to be tattooed. Now THAT makes any tattoo artists day!