The best RPs IMO are pretty much what Sheath and RedGajin said. However, I'd say the advantage to NOT having the same players every time as a determined group is you get the elements of the same personality every time, but however with this community, real, dedicated RP-ers are very hard to come by. There are varieties of people coming in, but most of them see the vast stuffty RP community, rather than a good RP. In this case, getting a small, dedicated group is really the only plausible action.
Besides that, starting topics on this forum is a good idea. For example: EgyptQuest, even though not easy to moderate, was rather popular and got around 30 people on a server at once. It was more of a puzzle, but it had the elements of an RP and had teamwork.
TL;DR I mostly agreed with Sheath and RedGajin.