Plan a design that you (or the person getting the tattoo) will have very little possibility of regretting. This is the first piece of advice everyone receives when they start talking about getting a tattoo, but too often it is ignored. What is important in your life right now may not be as important to you just five years from now; choose wisely, and avoid neck, knuckle and face tattoos if there's any chance you'll ever have to interview for a job that isn't at a tattoo shop.
Outline your design at about the size you or the person getting the tattoo will want it to be when it is actually on the skin. A good tattoo shop will be able to re-size the artwork if necessary, but think about what will happen if you draw a large, detailed picture that has to be shrunk down. The details will disappear and look messy, and bold lines will become thin. The same thing also happens if you have to enlarge a small drawing: detail vanishes.
Fill in and shade according to the tattoo recipient's pain threshold. The more area that needs to be filled in or shaded on the tattoo, the more time the tattoo recipient will spend under the needle. Also, very large, complex tattoos can take hours to complete, and some tattoo artists use techniques that drag the needle on the skin for shading. Keep that in mind if you are drawing for someone who may not deal well with pain.
Add detail sparingly. Skin is an organ of the body that changes over time; even the boldest of tattoos will fade and blur, and that means that detailed tattoos will become unrecognizable even faster. Try to stick to one or two basic, bold concepts to ensure that the work will still be recognizable for years to come.