I've always used Super Glue and have had good luck with that. Also have read about folks using something called E6000 which is available at craft stores. Never tried that one so I can't say how it works.
I used to make jewelry (the beaded kind) and I used E6000 to glue flat back stones to post earrings, pin backs, etc. It works like a charm and you'll be very satisfied w/the results.
Did you end up getting the E6000? I have some and it only seems to work (for me) in certain situations. Sometimes it never seems to "set"...I don't know why. Wondering how successful others have been with it.