I previously answered that I like the non-adhesive heatshrink tubing, but have been trying Npod's entertaining yet serious recommendation from a separate thread on something I didn't even know existed -- bondage tape. Yes, that kind of bondage. If you're the kind of clencher than deforms Vulcanite, a softy bit will be better, but for most folks, the bondage tape is fantastic. There are several advantages over the heatshrink tubing. It's very shiny black, so it looks really good on black stems. You can apply as much as you like, so it can extend further down the stem and you can give addition wraps if you want more protection. It's quite thin, so I use two or three winds around the stem.
The material gives you more grip in your teeth, so I find I'm not biting nearly as hard to get a firm, comfortable grip, so for me, I don't believe I need a lot of thickness. That also means it doesn't feel odd in my mouth. It stays in place without unraveling even though it lacks any adhesive, which of course is what makes it attractive for its designed purpose.
I got mine at Lover's Lane. It's a substantial quantity for about $12. Npod said he got his at Spencer Gifts. Since it's the same width as duct tape, I need to cut it lengthwise to wrap neatly and sometimes I get a little wrinkle or bubble, but I don't worry much about that. It's very flexible and can even conform to the profile of the button. By the way, it's flavorless.