I had heard that braining is just a myth that in't proven to help OR not help. :s
You can also try heating up a bowl of hot water- not boiling, but still hot to the touch. Dunk the whole (thawed) rat in it for 10 seconds, and then dunk only the head in it for another 10 seconds, so that the head is the warmest part of the body... offer it immediately. Some snakes seem to find a hot, wet rat more appetizing? Something to try.
Another thing to try is getting some dirty mouse aspen from the reptile store who has feeder mice... used aspen with some mouse poop and heavy scent in it, of course... and rolling the rat around in that first. Some snakes find mice a LOT more appetizing than rats. I roll the wet rat around in it after dunking in the hot water.