[@Saarebas] Well, the one game I'm trying to GM right now is plodding along at best, but I've run successful games in the past and I think the number one thing is that you need to do everything you can to engage the players.  I've seen RPs with great world building and plots die before because people had no idea what to do.  I wrote an RP that lasted for 2416 posts (my personal best for an RP if that isn't obvious) and I wasn't even that great at writing yet, but I did my best to intimately get to know the characters and gave them all subplots to go with the main plot.  

It probably doesn't hurt that I also picked a popular property (Fire Emblem) and gave them a story that you'd actually expect from such an RP.