I've used people, a come-along, a chain to another vehicle, but what I like best was a truck with a front mounted hitch and winch.
I'd set the trailer on the front hitch, and then use the winch to drag the load onto the trailer. It was a great arrangement if you were doing a one-man load opperation.
Unfortunately my dad sold both the trailer and the truck with the winch about 30 years ago.