As far as I have seen you are going to get a lot of support on this forum for Cervical Dislocation. Its a quick method and as or more human than any other. Certainly no complaints from me. I have tried cervical dislocation once with instruction from someone skilled at it. I failed and had to have the expert step in. That experience pretty much soured me on CD. That being said I wish it hadn't. It would be far easier than CO2 to dispatch a single mouse (My California King is the worst hunter I have ever seen, an average of 5 or more missed strikes before finally catching a foot or tail.) So for what its worth I am a big fan of CD when done correctly. I wish I could do it correctly. Until then a tank of CO2 purchased from my local welding supply store has served me well.
I wasn't too great at it at first either. Maybe you could practice on dead rats? It really is super easy and quick once you learn what you're doing.