Make sure everyone knows the posting rate you would like to achieve before they sign up.
Similar to this is: make sure one character who can post 5 times a day doesn't monopolise the story thread, when you've only asked for 2 posts a week.
And again: don't burn yourself out as DM by starting off with a high rate of posting you can't keep up over the long term.
Define what sort of a game it is, right from the start. Are you looking for deep character development, or is this a 'Hack and slash, who cares about plot?', sort of game?
Make sure you don't get any rules lawyers as players.
Here's a good one: If the party thief openly posts that he pockets items of jewellery and possible magic items before passing the contents of the chest to the rest of the group - expect the party mage to find an excuse to turn the little punk into a frog. Yes, I was that mage and no, I have not yet taken my revenge in character. But does that sort of thing fester like a gangrenous wound? Oh yes.
If you let the players communicate with each other by email, insist you get copied in. Then when a player leaves because of a mortal personal insult, you'll know about it before it tears the group apart.
Oh yes - and let us know what the setting is, I might want to sign up! 8)