I bought a rabbit distress caller, its a small metal circle with a hole in the middle, you simply put your mouth over and blow. Works great! it works good for foxes aswell. I also use it for getting the rabbits out the holes, sit around 20 yards from the set and call it for around 20 seconds and they pop their heads out giving a clear headshot depending if there are any in there but if a rab has its head down not giving a clear shot a simple kissing like noise with your mouth works great like the posts above,
hope this helps regards,
Cal