Remove the tires. As an added precaution against theft, be sure to remove the hub nuts and store them with the tires. If possible, store the boat in the backyard or in the garage so that it can't be seen by passers-by. If the boat has to be stored in the driveway, don't leave the trailer hitch facing the street. Even if the hitch is locked or has been removed, professional thieves carry coupler devices that can be quickly attached to the trailer.
Locks are available for props and some model outdrives.
Smaller outboards should be taken off and stored in the garage. The more you take off of your boat, the less attractive it will be to a thief. Take all valuable equipment and documents off of the boat for the winter. Small motors, seats, and spray dodgers are all candidates for indoor storage. Remove the trailer's license plate and, if possible, the tail lights.