I've switched all 6 of my snakes and haven't had to brain or pre-kill anything.

Make sure it's nice and hot. When I thawed in water I would usually finish off by putting the water from the sink as hot as it gets, and submerging the rodent in a bag in the water for about 15 sec and offering RIGHT AWAY after pulling it out. They drop back to room temp. pretty quickly, at which point it is unlikely that the snake will recognize it as food. They hunt mainly off heat signature.

If you have an infrared temp. gun I have had the best success when the head is about 110 degrees.

Another tip is to hold the prey by the scruff rather than dangle it by its tail or foot. Hold it low to the ground like it's actually moving around on the ground.

I've also been told by multiple sources that switching a snake that's an adult to F/T can be much harder than a baby when it's in a growth phase. How much does your snake weigh?