Hitch locks are an effective way of securing your caravan and preventing opportunist theft. Designed to lock over the coupling handle, and still enabling the stabiliser handle to be lowered, the hitch lock can be fixed in place when your caravan and tow vehicle are parked but connected (never travel with the hitch lock in place as this could hamper the emergency services in the event of an accident). When not coupled up, insert the safety ball within the coupling head before applying the hitch lock to prevent the caravan from being coupled to another vehicle. Al-Ko and Milenco are 2 of the most common hitch locks, both of which are recognised by most insurance companies and should entitle you to a discount on your insurance premium (most insurance companies insist on you using a hitch lock when the caravan is not hitched to your car).
See them in use in these videos.
Removing an Alko Hitchlock from a caravan
Milenco Alko Hitchlock