O-rings are little rubber plumbing gaskets that some restorers use. You pull your extra skin up over the glans and through the gasket. You can stack multiple rings one on top of the other to increase tension as you grow more skin (kinda like the way those African women do with the rings around the neck). Keepers are little plugs you can stick into the end of your trapped skin to keep the rings from popping off. You can buy o-rings at any hardware store for just a few cents each. You need to experiment with different size rings to find the right one for you... one that holds good without pinching the skin. You have to have quite a bit of slack skin to use them. I have tried, but even at six months, I don't have enough slack to keep even one ring on.
Please note: the rings do not go on the penis itself. You have to pull your slack skin up and past the glans and then put it around the skin only, not the shaft or head.
Pros: O-rings are good for fast skin growth and reputedly put a nice, natural taper into the end of your fauxskin.
Cons: difficult to place sometimes, have to have some slack skin first, pop-offs without keeper. I would not use them during sleep; potential circulatory danger from tightness, pinching.