Yeah you don't hear about stolen trailers around here too often. When you do its because the trailer was left in a dark kind of isolated/ or easy to load up (with out being noticed) and haul away spot.
When I boarded I kept my hitch locked and my trailer locked up tight. But honestly if someone is that determined, they will steal it no matter what you do to prevent. The best you can do is make it as unappealing as possible.