Cockrings can be very helpfull in improving and maintaining your erection. Most are designed to fit around the cock and balls, such as the snap-on ones. The solid ones are also worn that way, by first putting your balls through one at a time, then snaking your cock through. However, I find the most effective type is a simple ring slid over just the shaft, pushed down to the base. I discovered this when fooling around with an ex-boyfriend's ring. The ring that fit around his cock and balls was a very tight fit on just my shaft. It got me quite hard when pushed down to the base, and after wearing it for a while my girth begain to swell considerably beyond its already thick measure. When the BF saw it, he just had to get fucked then and there, and sat on it, but unfortunately he hurt himself by taking it too quickly.
That was a metal ring, and proved rather difficult to get off later, due to the swelling of my cock. Therefore, to avoid getting stuck, I suggest using a rubber ring, which can be easilly cut off if need be. The rubber ring needs to be a bit on the small side, as it will stretch. The best way to get it on is to use lube, and slide it over your cock before you are really erect. That way, the ring will be nice and snug as you reach full girth when you are erect. Make sure not to use a very thin ring, as it can become painful. Better hardware stores often carry a good assortment of different sizes of rubber O-rings in the plumbing section, for about 40 cents each.
Also, do not fall asleep with a cock ring on, or keep it otherwise on too long. Once at IML (International Mr. Leather) I had on a very heavy chrome ring around my cock and balls. I had just bought it, and think it may have been too small. (But it did give a hell of a package in my jeans.) I went to bed late, and was having trouble getting it off, so I decided to deal with it in the morning. Well, I woke up about dawn with my cock horribly purple and swollen, and the ring really stuck. I tried a cold shower, but that made the balls tighten up, which only made things worse. I wasn't sure what to do in this case- does one call 911, the concierge, or (at a fuckfest like IML) just knock on doors to get help?
Fortunately, I had gotten a lot of free samples of some very good lube. By greasing everything up good, and winding a bootlace around my cock to compress it, I was able to ease the thing off in about 20 minutes. Since then, I have been very careful about not keeping metal rings on too long.