Patience is a big key. Both as you are dangling a rat in their face hoping they'll take it, and also in not giving up after the first attempt fails.

I think the most important thing is to make the sure the head of the prey item is warm enough. Lots of people have had good success by blasting the head with a blow dryer just before offering it. I've had great success by dipping the head in VERY hot water and then offering it to the snake. Lots of BPs will take it off the floor of the cage, but some need you to do the "zombie rat dance", especially while you are converting them.