I usually just let the mouse thaw overnight then warm it up right beforehand. Kara suggested a hair dryer. I've also read that people hold it next to a light bulb for a few minutes to warm it. I have a small heat pad on the top of his tub, so I set it on that for about 10 minutes beforehand.

Granted, I don't feed multiple snakes multiple mice...but thawing them out overnight instead of using the hot water method might be easier for you?