[b]Step 1:[/b] Understand the audience.
[i]Grand Scale:[/i] Figure out what RPG likes and cater to it for a larger potential audience. (ex: RPG loves Fantasy stories, they've always been a popular choice, especially if you have no name as a GM yet.)
[i]Local scale:[/i] Once you have players in whatever RP you decided to make, ensure they feel (and to a certain extent, [i]are[/i]) involved in creating the world. Encourage them to tie their back stories into certain events you mention in the plot, or if it's total open world with no end goal, to mention friends, family, locations, personal goals and failures, etc. [b]Then keep this momentum going[/b], keep the OOC chatter alive or get everyone together on a messenger client so they can be chatty. Bonus points for IM's being that if they're even remotely active on them you can ask them in real time how their post is going, or when they can post.

[b]Step 2:[/b] Momentum. I can't emphasize this point enough, so it gets its own step altogether. Keep pushing forward, keep making everyone feel important and useful, keep the creative juices flowing. It's like a wheel, a big wheel. When it's moving, the pace will blow your mind. When it slows down everything gets harder to keep it motion and when it stops it takes a lot of effort to get it going again. You don't have to move at the stupid-fast pace of the free section, but trying to keep at least one post a week is a good idea.

[b]Step 3:[/b] The RP is never dead until everyone gives up on it. If there's been no post for over a month, and everyone still says they have interest, then keep posting in it anyway. There's no rule anywhere that says that after X amount of time your RP is dead. Do what you want and never feel pressured that if you've lost momentum, the RP is dead... Because it isn't. It's just very sluggish now. :hehe

[b]Step 4:[/b] Use the information players provide in their sheets to your advantage. Character X has a sister? Introduce her into the story in some manner. This directly tells the player that yes, their character matters, their history matters, what they wrote matters and is part of your universe.

[b]Step 5:[/b] Got momentum? Have the players directly involved? Writing an RP that caters to the overall collective interest of RPG in some way? Understand that an RP is never dead so long as people want to post in it? Congratulations, this, plus a couple of attempts to get an idea going with some consistent players in it, will pretty much guarantee that you will have a long lasting RP.

The only question is if you really want that.