I've HEARD.... 

If you tie a rope to a post just outside the trailer and then run it through a front side gap you can then hook it to the end of the rope that is on the animal. This allows you to pull forward really slowly and pull the animal onto the trailer without help. Before you start this rodeo, make sure the post isn't nearly rotted off!

It's much easier, but much less entertaining, if they're dead or heavily sedated before you start.