1) Research the artist, you should know what you are getting into before going in. Portfolios mainly, take a look at their work to make sure what they do matches what you want. Also look for completed pieces that are done on your skin type or ask to see examples. It's possible that artists who have lots of photos on lighter skin don't have much experience on darker skin and may not be able to deliver the same quality.
2) Help the artist help you. Getting reference images is always a plus, even for freehand artists. Gives them a nice starting point. pictures of other people's tattoos is not a good reference...pictures from life and such are. Google is your friend.
3) Make sure you are being realistic. You can't put a million things in a small spot. Make sure there is room for you tattoo.
4) Keep distractions to a minimum. Don't bring an army of people. You want the artist to feel comfortable with you. An ipod and a friend are more than enough.
5) Be mindful of healing. If you know that the tattoo is gonna rub against something or you will be in the sun or water for prolonged periods of time afterwards then you may want to wait to get the tattoo.