Carbon dioxide is another humane method, it puts them to sleep easier than in the freezer. I've read studies that some herps' brains do not shut down during freezing before they can feel their cells burst from the ice crystals. They based this finding on the amount of cortisol (hormone released during stress) found in the bloodstream after freezing as opposed to before. It's a controversial finding and I haven't read anything recently so it may have been discounted.
You can buy cylinders of C02 and a regulator made for making seltzer water at most restaurant supply houses. Just use a plastic bag to hold the gas in instead of sending the gas into the bottle of water as you would for seltzer.
It comes in very handy for fresh-killed rodents in a humane fashion as well.
The Zombie Mama is here!