Securing a bike trailer
It depends a bit on what type of theft you want to prevent.

If you want to prevent opportunistic theft, then all that is needed is to put a lock through the wheels, as a thief would then decide that the trouble of breaking them isn't worth his effort.

If you want to prevent deliberate theft, then adding a clamp to the towbar and locking it with that to something fixed might work, although no lock can withstand a determined thief.

Here is a list of locks, with scores for how good they withstood some tests :
http://www.stichtingart.nl/sloten_resultaat.asp

Although the site is in Dutch, I think you can use it.