I agree with everything Alethan says. Players are like cats, and a good DM needs to be adept at herding them. Set an expected posting rate, stick to it when possible and don't be afraid to prod or move things along if no one is moving things forward.
About two years ago one of the moderators of a forum where I am primarily active (rpg.net) instituted a Blacklist for pbp games that seems to have worked fairly well at, if not increasing the length and duration of games, than encouraging people to be more polite about it. Essentially, if you have a player or DM that has a habit of ghosting -- either staying active on the boards but not posting in games they're already in or disappearing altogether, leaving folks hanging and reappearing some time later looking to start up a new game and repeating the cycle -- the members are encouraged to report that individual to the mod and they would be given a Blacklist warning in the specific thread. It's not an official sanction, or a banning, just a note of the fact that they ghosted on a commitment. So, if someone is starting up a game and someone they've never played with wants to sign up it's a simple matter to check the Blacklist and see their record.
There were some people who objected -- who thought it was a fascist move -- but we have found the following:
1)It encourages people to speak up when they can't handle something, as opposed to just dropping out. Saying "sorry guys, I thought I could handle the time commitment but I've realized I can't" or "sorry, I'm going to have to drop because this isn't the style of game I was looking for" is much better -- though harder, somehow -- than simply stopping to post. So, it has certainly encouraged politeness.
2)Many of the Blacklist warnings have gone to those who most strenuously objected to the concept, not surprisingly, but it is also important to note that the frequency of warnings has dropped off significantly.
I'm not suggesting that US institute such a thing -- it means a lot more work for the mods, for one -- but just wanted to mention it as a possible solution. You can see the Blacklist here:
http://forum.rpg.net/showthread.php?709 ... -Blacklist
Note that you have to be logged in to rpg.net as a member to view that particular forum.