The last months I've been using rubber grommets from Grolsch beer flip top bottles and the plastic bag closers from bread bags as ways to lock straps to my guitars. They work well. Whereas the straps would slip off the button of either my acoustic Martin or my Gibson electric about once a week once I started using the rubber grommets on the Martin and the plastic bag closers on the Gibson neither strap has slipped off. Ever. But, although the plastic closers are readily available, I can't find Grolsch beer in the super metropolis of Missoula, MT (pop. ~ 69,000) and I haven't dropped by the local DIY brewing store or otherwise found additional rubber grommets to supplement the three I have. I thought eventually I'll need more but less than the quantities I offered online.
Then I acquired an Ovation 12-string and started thinking of a more 'proper' way of securing straps to the guitars. Ultimately I purchased a Lock It strap, http://lockitstraps.com/strap (3" black poly bass strap, about $25 on EBay, Amazon, etc.). This is pretty cool, IMO. It is very secure and pretty convenient. It works like it was made for the Gibson - click...click... and it's on. The Martin has a larger strap button and it's a little bit more of an effort to slide the strap over the button. As the leather of the strap stretches it will become easier/quicker yet because of the design of the 'lock' the strap will be just as secure. The Ovation has an even larger button...it's huge in comparison to the other two guitars. None the less, I can work the Lock It strap over it and all else works as designed.
In my search for locking straps I found lots of comments about straps not fitting guitars, marring surfaces and other disappointments. What I can say is that the Lock It fits my three very different guitars and it has not left any marks. IMO, the strap is very secure and simply won't slip off.
It's not as cheap a solution I was looking for but on the other hand I have a new, comfortable strap that I like and it requires no modifications to the guitars.
"It takes a lot of devotion and work, or maybe I should say play, because if you love it, that's what it amounts to. I haven't found any shortcuts, and I've been looking for a long time."
-- Chet Atkins
-- Chet Atkins