I recommend getting a kitten style H harness. I find the rabbit ones pet stores sell are a little stiff, the ones they sell at Wal-Mart for kittens are made of a softer material and are just as adjustable, as well as the clips being a little stronger which is always good to have it more secure.
I know some people use the vest harnesses and while I can see why some may prefer them, I do think it is more constricting for the bunny. But they work when they are properly adjusted.
These are the type of harness I use,
They are great little harnesses, they are secure and distribute the pressure from the leash.
Just make sure you DO NOT get a figure 8 harness!
Anything that is not the vest or the H is NOT safe for rabbits. This can tighten and choke the rabbit.
As always, be careful when using a harness, some rabbits do not respond well to them and if you don't know how to use them, they can break their backs. Be EXTREMELY careful. If you know how to use them they're fantastic for getting the bunnies outside for fresh air.
We even actually have a video on introducing the harness...
http://www.youtube.com/watch?feature=player_embedded&v=SiOLgP-bU9w