Hi Steve,

Ouch, i've done that too, it's the worst feeling..."oh, I'm sorry guitar!!" is my feeling after.
A quick fix is to use a bread tie from a loaf of bread, the little square plastic ones? and while your strap is on just clip it over your strap peg and I should keep your guitar from slipping off.
This tip is featured in one of our new GT Channel episodes as the tip of the week.

Rock on! :D