[QUOTE=Old Breadbutt] I think egg beaters are the best. 4 sides to engage on and if someone tries to ride off on your bike, they're going to have a tough time with hardly anything to stand on.QUOTE]
I agree, esp w/ the tough to ride w/out proper shoes. I have tried and almost hurt myself too many times.

I had some bad experiences mtb'ing w/ SPDs. foot would just pull out of the pedal. Switched to EggBeaters a couple years ago, and haven't looked back. They are on the mtb, and both fixies.

As for practice...if you (or someone you know)has a trainer, you can set up on the trainer and get used to clipping in and out. Then ride around in an open park (ie. soccer field) for a while, clip in and out a lot. if you fall over, won't hurt near as much as pavement does.

Good luck!