As somebody else mentioned, the best method is to buy frozen food and feed them. It's cheaper, it's easier (at least you won't have to kill them), and it generally means a bit milder temperament for the snake.
If you must feed live, there is one trick I recommend. It's called the cervical snap. I don't like cruelty so this is the most humane (and non-bloody) way I've ever found. It works great with mice but not so easy with rats.
With mice, I personally pick them up by the tail, put my hemostats around their neck, and pull. It separates their spine at the base of their neck and it's over. No screeching, no blood, and it's quick. Another way is to hold one by the tail and let them try to run away. Take a pencil, ruler, hemostats, screwdriver, etc., and pin their neck down to the surface (usually a table I suppose...I'd recommend newspaper or something else on the table). Press hard and then pull their tail. Same result.
It is snapping their neck, but at least it's not real horrible and it doesn't require any smacking or slamming 
Good luck,
Jason @ Jason's Jungle