Velcro when they are tiny and cannot pull them off, and over the head once they are bigger! Make sure that the hole is not too tight though.

I love the kind with the stretchy hole over the head. They're quickest to get on and off!

I'm a big fan of poppers (snaps) or over the head-velcro seems to irritate the back of the neck (unless you make the front-side closure kind, then velcro is ok). Good luck!