I guess I can give a truly sincere answer now.
Three simple steps to help stop the whole "RP's not making it" problem.
#1: Always take in twice as many as you need. Half of them will drop out in the three stages: Interest, character generation, first few posts.
#2: Always directly interact with and entertain your players. As a GM, you are just as much an entertainer as you are a leader. You have to engage the players in more than just telling them to post, otherwise, they'll forget. (Not saying that as an elitist GM--as a player in The Library, I sometimes forget to post myself. It simply happens.)
#3: Accept that your RP will die.
...Then go take the people who stuck with it to the end, ask them what you could have done better, tweak it, re-release it, reinvite the people who stuck it through to the end, don't accept the people who randomly dropped out on you again, and watch its survival chances increase by about 500%.
Seriously. Once you have the dedicated base of two or more players, time to refine your idea from a failure, and then you proceed to actively engage your players and always take in more than you need knowing you will lose some people? It's pretty much as good a chance as any RP is ever going to get for long-term survival.
Examples of this concept in action include Legend of Renalta, Stellar Empires, The IEF Venture, The Library, Apocalyptica, and the list goes on, and on, and on...
If people want direct links to the RP's in question, let me know.