Insurance!!!!
Seriously, I have a hitch lock and some chain in various places.
But they are just a deterent if they want it they will get it
!
I watched a video on a boating forum from a guys security camera when his boat got nicked and the guy had -
1) A wheel lock (A full on on one not a cheapy)
2) A hitch lock
3) Chain
4) Lowered his motor to the ground
Two blokes walk up, and in instant each of them have the locks in pieces and one guy is raising the motor whilst the other rolls the trailer out of the driveway.
Took them about 40 seconds to be rolling, you couldn't see what they did to remove it as it was crappy security camera but you can see each of them take a piece of.
Pretty scary to watch, the video got removed as I think they got the guys of descriptions but since then I put a few a things on but most off all make sure the van is insured.
Some people have been putting in GPS trackers and a variety of other anti theft measures like custom stickers etc etc.
There is a good read here with lots of informative posts and information -
http://www.expandasdownunder.com/threads/expanda-security.1018/