Actually, the most humane way, next to carbon monoxide, according to animal protection organizations such as the RSPCA, is too squeeze the rodents brain out through the back of the skull. Instant death and no pain... Dont do it unless you know how, though.. If you do it the wrong way, obviously you are going to cause the animal considerable pain. Your vet should know how to do it, so next time you see them ask them too show you.
For gods sake, DONT FREEZE THEM!!! Are you aware Duke, that the bloodvessels burst while the animal is still alive when you freeze them? It is obviously very painful, and takes a very long time. If you dont have the balls to squeeze out their brains, or knock them against a wall (lets face it: most of us dont! I used to hate it, but now it doesnt bother me because I now know it is the most humane way) invest in carbon monoxide.. If you dont have the money to set an appropiate chamber up, you are cruel.