Your best bet is a small fence, made of chicken wire or hardware screen. If beauty is important, you could install a small ornamental fence (such as a picket fence) and attach the protective screening to the back of the fence so rabbits can't slip through.
There are also some good repellents on the market. Google Messina Wildlife and Liquid Fence for two examples.
Finally, I always suggest seeding your lawn with white clover, because rabbits will go for that over almost all other things in your garden. At least that has been my experience.
Luke Miller, editor
Garden Ideas & Outdoor Living magazine