I learned this from VPI and it worked on some of my most stubborn. Hold the rat by the neck with forceps and put behind the water bowl. Have it "peek" out from behind the water bowl and as soon as the snake flicks it's tong, have it "hide" behind the water bowl. Do this several times until the snake starts to approach and then have it "walk" by the snake and "bam!" it should strike at it. As the others have said, make sure it's warm too.