I use a 1/4" strip of bicycle innertube, do a loop similar to Msilva's picture and put the mouthpiece through there. I've found that it works the best of any method that I've tried, plus it's going to give (or break) if I really need access to the octo or someone sneaks up on me and grabs it. I keep multiple strips around the end of my webbing so if one breaks (which the first one has yet to do) I can switch it out underwater if need be.