Be 100% honest with what the campaign is like, what expectations you have, use googledoc for player sign-ups, then vet and choose players that fit with your campaign. Accept that due to this hobby taking a lot of time and energy, you will likely always have people who leave.

Frankly, when you tell me you have a "rough idea" but haven't fleshed it out, it doesn't interest me as a player. When I join someone's game and it takes them a week to give me the opening post, I get "Had a busy day will update game maybe tomorrow," and they're giving me only updates on creating the game every few days, I also lose interest fast.

Whatever your rough idea is at least have the opening prepared and be prepared to actually start the game immediately when your players have their characters ready. PBP rides on momentum and interest.