I agree with the idea of games for building interest. On OldGuild, I used private Groups as playgrounds for my RP's. We had a group discussion for music and our characters' theme songs, short stories about days from our characters' pasts (Most embarrassing day, happiest day, etc.), a neat thread where we listed all the [url=http://tvtropes.org/pmwiki/pmwiki.php/Main/HomePage]tropes[/url] our characters fit into, collaborations that would later be edited into the main IC, all sorts of neat stuff. (Please, O Great and Powerful Mahz, I need my Groups back!) Those are all great interest/activity builders, as well as nice character development exercises.

Yes, if you are a GM it is mandatory that you be active in the IC and OOC. Without you, the players are lost. I'm currently struggling with Twisted because I wasn't posting much for a couple weeks while I changed jobs. It nearly cost me the roleplay, but thankfully I'm getting some players coming in on the recruiting thread... and I actually designed the game to operate around the fact that during its run, we'd have players come and go. It is what it is: people's lives change, they don't have as much time as they thought they would.

I also agree that you have to outline your post expectations and stick to them. You just have to keep in mind that you'll set the pace for your RP with your post limits. If you have your players posting every day, you'll eat up your whole storyline in four months, and your game is done. If you post once every couple months, your RP will take forever to get anywhere and many years to complete. I'm not saying either is bad, it's just something you want to be aware of. Twisted is long-term, so my post expectation there is once every two weeks. I've been in roleplays with shorter and longer limits... but there pretty much always is one. It's important for players to know upfront what the consequences will be if they don't post. Will you wait two weeks to replace them, or two months?

Begin with the end in mind. When you write a roleplay, have a resolution for your plot and at least a vague outline of how you want to get there. Be flexible, because your players can and will throw curveballs at you sometimes, but if you don't know where you're going there's no telling where you'll end up! If you get lost as the GM, your players usually won't find a way on their own. They'll give up and move on to a new game.

One thing I can think of that I didn't see mentioned previously is to be mindful of the seasons. I've been on RPG for four years now, and it's always busier around here when school is out for the summer. I'm not trying to be derogatory to students in any way, but the simple fact of life is that when they return to school their studies often leave them too busy for roleplays. Recruit whenever you have to, but I would advise starting your RP in fall or spring.

Another piece of advice that has served me well is to recruit more players than you actually need when you start the game. If you need eight, start with ten. That way you can lose two players and be right where you wanted to be three months down the road. 

Even with all those tips and tricks, sometimes your RP's will still die. Hope some of this helps someone... somewhere. XD