I've heard of many tricks, but the one that seems to ALWAYS work for me is this:

No matter what size snake it is, take a live fuzzy rat/rat pup (which is young enough where it can't cause any damage to your snake) and leave it in with your trouble-eater snake for about 30 minutes. You can either put the snake and rodent in a smaller tub all by themselves, or just leave the rodent in the snake's cage. You can turn off the lights if you want to add some security. Close the cage and check back 30 minutes later.

I've never had any extremely picky feeders here, but doing that always gets my snakes to start eating again. I just did that tonight to get snakes off of their winter fast, and it worked like a charm!