I always wonder why, when people walk into a tattoo shop, the first thing they go to is all the flash on the walls or in the books, instead of the artist's personal portfolio. You may not find what you're looking for in a tattoo in the portfolio, but it will definitely help you decide if you might want to be trying to find the exit door! And don't be swayed by pretty pictures. Really look at the designs up close. Scrutinize the line work. Are the lines straight and smooth? Do the circles look like circles? What about the coloring? Are the colors blended well to create even forms of shading, dimension, and depth? All these are things to consider.
A sidepoint to keep in mind, though: remember that photos can be stolen from other artists. If you have the time to spend, stay and watch the artist you are considering.
Don't be shy to ask questions! Trust your instincts, and always remember......If in doubt, don't!
http://tattoo.about.com/cs/beginners/a/blportolios.htm
