This is one of the reasons I'm more interested in building community than merely joining a site and jumping into a role-play only to watch it die. I think it's really important that you role-play with people who're your friends and comrades. It creates the additional motivation and zeal that inspires people to keep writing and not just abandon a project from lack of interest or due to 'stuff coming up'. It's a lot better to role-play with friends, much in the same way it's better to do any hobby with people who're close to you.

All the best role-playing experiences I have that I'd care to name involving me and my fiancé or me/us and some of my closer friends who are privy to enough information about my main story project to be able to role-play in that setting. The entire reason I joined this site at all was due to losing contact with some of them due to life problems getting in the way for them -- real problems, just for the record, not the standard 'oh I got a little busy' bullshit you hear from people when they disappear due to lack of interest or lack of inspiration. The role-plays I involved myself in with these people lasted many years.

Really, the most important thing in my eyes is to form these relationships before role-playing when possible.