what you need help with? The Play by play part of it? the acutal dm'n?
Tips here is to set up a situation, give the party a chance to respond and if someone doesn't respond in due time (set it up ahead of time) then proceed onward with the PC's that have responded.
Also you the DM roll the dice, do it iall your self, it saves time and effort in this.
Try coming up with a good theme and mix and get allot of background ready to be used or be able to be created ahed of time.
Come up with a few reasons why people would be where they are, some info on the world if it's a home brew, and a few cities.
Pick what classes, races, books to allow, this cuts down on confusion.
Set a delay of posting, if X time has gone by without a posting then we move onward.
Set a delay of people not posting in a long time then they are out of the game and call in substitutes. It's a fact people grow bored and drop out so make sure they can be easily subbed in and out at a whim...
that's all i can think of, i haven't run any PbP (but have an online game) but these are the rules i see followed and what i would do as a DM.