We actually have a whole section for that!
Our article and guide section!But from my experience, a lot of it is in how you present your idea. In a way, think of it like submitting a resume of sorts; it should be eye catching, easy to follow visually, and outline exactly what the game's about, what's being offered, and what's expected. One thing that's very important is that the GM appears to be confident and as a GM, that you believe wholly in your ability to run the game and that you can put together a story. Plan it out before hand! I can't stress that enough; try to work out the plot and what you want to happen, NPCs, and write it all down. Definitely figure out what kind of game you want to run, and how much control players have over what happens. You don't want to stifle player creativity and choice, but you also want to keep them moving down the story's path and you need to be to adapt to stuff you didn't anticipate.
One VERY important thing that I notice is a common trend in all the long-lasting games is a healthy OOC where people feel comfortable with one another and can just goof around and have fun. The more people get to know each other and feel like they belong in a game, they're much more likely to remain in a game. Have a sense of humour, keep engaged with your players, and definitely listen to them!
Overall, the best thing you can do is prepare the game well in advance and believe in what you're doing. It's very obvious when somebody makes an RP on a whim, and a lot of times, that ends in failure because the GM loses interest or doesn't really know where to go once the initial excitement dies off.
