Don't tell me the moon is shining; show me the glint of light on broken glass -Anton Chekhov
Total play-by-post addict, PbP speed runner (not really anymore, I aim for daily consistency instead!), GM of the Red Arm of Absalom.
Why a fast PbP works:
When people join a PbP, they're excited. They can't wait to start. If their characters are brand new, this is doubly true. As soon as the GM drops the first introduction post, every player usually posts within the same day.
The first week into a PbP has a similar level of posting going on. So long as the GM keeps up their daily post, the players will too.
By the second week, the scenario may have hit some roadblocks. Players won't post unless the direction is somewhat clear. There's a world of difference between tabletop play and play-by-post. There's no side dialogue players can quickly have with each other on these forums, so it isn't always clear what the party's intentions are. Maybe one person wants to talk to the NPC and the other wants to kill her, but neither of them want to step on each other's toes.
Days pass. No one has posted. You post again, hoping to get someone to post. One of the more active players posts. No one else does. Another day passes. You lose interest because if the players don't care about the game, why should you?
That's the very reason why I love fast PbP games. Everyone is constantly checking, updating, roleplaying, and having fun. If you can establish an understanding with your group that more than 2 or 3 posts per day are needed, I can guarantee you that your group will be a lot more interested and effective. That, and they'll blast through a scenario so quickly, not only will completing it feel like an accomplishment, doing it in such a short amount of time will!
My goals as a PbP GM:
1. Guide your players. Drop hints. No one remembers what that handout said that you showed to them four days ago. Just because you're invested doesn't mean they are, so repeat certain key points from the handouts or NPCs. Repeat parts of the mission briefing or story if needed. Anything to get them back on track.
2. Go with it. Just because the scenario/PDF only indicates the players have to go to the temple, doesn't mean the players have to go the temple. Maybe they want to go to the bar first. Maybe they want to explore the town. Make up the bar scene, explain it, create an NPC and have them interact with her. Dress up the scene by adding a barfight or some other activity that won't interfere with the story but that will otherwise add some fun.
3. Foster PC relationships. When players begin to roleplay with each other, you know you've done a good job, and you know you've got a group that's going to stick around. You can't exactly force this, but you can certainly set it up so that PCs need to work together to achieve their goal - that is the very essence of PFS, after all.
4. Keep it fun. Did your PC forget to use their Heal skill on themselves when they could have avoided ability score damage? Would you let them correct that if playing on tabletop? I would. Stuff like this happens all the time. If you enforce the rules harshly, you take away from the game's fun.
5. Roleplay. Pathfinder ROLEPLAYING game is what it's called. :)