Keep a notebook or sketchbook with you for several months and write down ideas for your tattoo. Look at other people's ink and make notes of what you like and don't like about it. Pay attention to how colors look on different skin tones and how images move on different parts of the body.
Make a list of themes you find running through your life, and think about how those themes would translate into images. A theme like "cats" will probably be easier to execute than, for example, "sadness" or "love." Think about concrete representations of abstract ideas and how to translate them into a tattoo that is unique to you.
Collect as many images as you can of styles you like, artists whose work you relate to and thematic material. The more visual information you present to your tattoo artist, the better. You may even wish to make sketches yourself, although you should be prepared for the artist to redraw them in a way that will make the best tattoo.
Visit as many tattoo shops as you can and look through the artists' portfolios. Find an artist who has done bodysuits in the past, or at least large-scale tattoos like sleeves and back pieces. Talk to the artist if possible to get a sense of what it would be like to spend a long amount of time with him. A bodysuit will require many, many hours of work, which for most people is spread over a number of years. You need to be with someone you trust, like and talk to easily.
Listen to your artist and trust that she knows what she's talking about and wants to help you properly execute your design. If she tells you that dragon will look better on your back than your armpit, it's probably true. Remember that tattoo artists are just that, artists. They are not copy machines, and they love it when you give them your respect, trust their judgment and let them do their jobs. You should never get a bodysuit from someone you don't trust to make valid artistic decisions.