I went to my local hippie store (sells herbs, jewelry, various pagan statues and dungeons & dragons type stuff) and bought a little kit with henna powder, a henna stencil and a henna cone and tip and instructions on how to use it all. The henna was probably a little old, but it still did the trick. My friend Sarah (mcsarahb here on MDC) was a month more pregnant than me but we hung out at her house and painted each other's bellies and sat out in the sun and let them dry on a random hot Saturday last April. They looked pretty cool and mine lasted just about 2 weeks before it faded.
Here's a pic of my belly with the henna paste still on. The longer you leave the paste on, the darker the the staining on the belly. I washed mine off later that day in the shower. I borrowed some books from the library to get some ideas about designs, but Sarah and I just made them up. It was fun.

~claudia