We've probably had at least two or three discussions on this already, but I'll add my two cents again, anyways!

I bought some Dunlop straplocks a couple years ago. What you need to do is remove (unscrew) your old strap buttons and then install in the now empty hole a socket-type thing. It's a little scary if you have to drill (I don't like the thought of permanently altering the body of my guitar).

Anyways, your then attach another piece to both ends of your strap. This piece has a little knob on it that fits into the socket that you just put into the strap button holes. The knob and the socket click together, and no amount of pulling/jumping/spinning your guitar over your head by the strap will make it come undone, unless you first push the release button.