Some good bait options for woodchucks are fresh fruits and vegetables like corn, cantaloupe and broccoli. Woodchucks also go for sunflower seeds and peanut butter.
Place the trap near one of the burrows in the yard. Burrows are identifiable by a mound of dirt in the yard. Find the entrance of the burrow and put the trap near it. This ensures the woodchuck finds the bait and trap after exiting the burrow. There are some commercial bait options, but fruits and vegetables tend to be the best. Spread some of the bait on the floor in front of the trap, then put the rest inside of it. This encourages the animal to enter as it enjoys the snacks.