I personally got several cockrings. (Have been meaning to do a thread on them ...)
Anyhow, Stillwantmore's tip is spot on in my opinion. Silicone ones are good. Definitely look for some width in the cockring.
After a
Bathmate session I usually wear a halved Doc Johnson cockcage ring. Mine have lasted quite well.
Other good option is Tantus Super Soft C Ring, also written about on this forum.
I would like to try the Horseshoe cockring, but so far the cost of one has put me down. However, on the pricier range I acquired myself Jnaja by Velv'Or. It's a whole package deal cockring.
There's one problem that's the same with Horseshoe cockring; the sizing. So, get your measurements right if you go for either one of those. (Or cockrings that are metal / non-stretching material.)
Oxballs also offers some nice cockrings, like Powerballs for example.
Lots of choices out there. The O-rings work as well, if you're really on a budget. I actually bought a box of different sized ones. With O-rings, as 2slow mentioned, width is to be taken into consideration. Most of O-rings available are on the thinner side, so they're a bit rough while they do work. Hairbands as mentioned, also work, just be sure to get one without a metal "hook" in the band. That rule should go for all hairbands though, that metal in the bands does damage to the hair.
heard uli's probably best option but dunno about 
