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I used to use the simple elastic Croakies (the ones with the tight fit, not the ones with a strap that's 10 times longer than needed), but many modern glasses are now so slender along the temple pieces and the part that curves behind the ear that slip-over sleeves just don't grip tightly enough. I even tried that while swimming, and rapidly raising my head from being submerged simply pulled the glasses free of the strap. Nowadays I just use string or parachute cord. The trick is getting the adjustment right.
For metal frames, I tie it tightly just in front of where the plastic for the ear piece is molded on. There's a slight "step" there and a tight knot can't get past that point. On "my particular head" (no guarantee that other heads will be the same, but they will probably be similar), if I make the string such a length that I can put the glasses on with the loop of string just snug over the top of my head, that loop can then be pulled down behind my head, low to the top of my neck, and the glasses can't slide forward more than half an inch or so but the string also is not tight unless the glasses get yanked by the water I'm swimming in. Perfect fit. The string in that position goes across my ears, but I'm used to that now.
For those who actually prefer those extra long foamy straps that float, have you ever actually tried to spot them floating down the river? You won't ever see them again unless someone in your group happens to paddle right alongside them later on. Oh, and I did that once. I found a partner's glasses floating down the river and grabbed the strap and pulled, and since the lenses provided a lot of water resistance when I pulled, the glasses simply slipped free and left me with an empty strap (same thing would happen if they'd have been worn on one's head at that moment too). Might as well not have spotted them at all.