I have a haul trailer that I keep in a storage facility for the past 10 years. I do two special things. It is tandem axle, I store it with one tire flat on each axle on opposite sides. These are good tires with low air that can be inflated with a porta-tank. I lock the jack in place with a heavy chain and lock.
The most important part is having it in a storage facility with a full-time 24hr real live security staff. It also helps that the trailer is older and not as appealing as the easier to steal new ones. Since I have been there, no trailer has been stolen off the lot.
Now where's that piece of wood I need to knock on????