Tips are a good way of showing your artist that you appreciate the work they did for you. The amount tipped for a tattoo is based on the amount and quality of work that has been done.
Remember that the tattoo you get will be with you for life. The more you are satisfied with the work, the more you should tip. For project tattoos that take several sessions, a gift is an acceptable tip for an artist. While you are at the shop, look around or ask questions about what your artist likes or needs. Art, tattoo and body modification books are great to give and are always appreciated (just make sure they don’t have the book first). Of course a cash tip is always the safest bet.