So I made this video for another thread, but I thought I'd post it here as well.
This video shows how I put on my gopro head mount in a way that prevents it from completely falling off underwater. It's free and only takes a second to add an extra level of protection to your gear. This method works by forcing the head mount to tangle with your snorkel whenever it gets knocked off your head. Once tangled the gopro head mount becomes nearly impossible to yank off.
Now this method works perfectly with the Riffe Stable snorkel, which has a specially designed clip to ensure that the snorkel is securely fashioned to your mask. However I wouldn't try this with a cheaper snorkel that only uses a rubber O-ring attachment. This method relies on your snorkel staying attached to your mask.