Apples, carrots, cabbage, and other fresh green vegetables are good baits in warmer weather or climates. These soft baits become mushy and ineffective once frozen. A good summer bait for garden traps is a cabbage leaf rolled tightly and held together by a toothpick. For best results, use baits that are similar to what the target rabbits are feeding on. This will also probably catch woodchucks, so be prepared.
Some believe that rabbit urine can also be an effective lure. This can be purchased at trapping supply companies. Be careful with urine as it may contain infectious agents. Wear latex or vinyl gloves when using to help reduce the risk of getting an infection.
Remember, many states require a permit before you can trap, relocate, or kill rabbits. Check with your wildlife agency before initiating any trapping program.
Resources: (online)