A play by post game, in which the game is played out through a series of correspondences between players, can be very tricky to get started or keep maintained. The pacing and level of progress that can be made is highly dependent on the availability and dedication of the players in your group. If one of them never gets around to contributing, or spends too much time taking actions, it can bring things to a screeching halt. That's why it is important to have safeguards in place to keep things moving if the game suddenly trails off.
To facilitate all of this, it is important to choose an effective medium for running a P-b-P game. Preferably a forum or e-mail list that notifies the players on a regular basis whenever someone posts to it. Players are going to be very busy and distracted by events in their everyday lives and are not going to always have the time or frame of mind to check in on an online game on a regular basis. They will need a way of being notified or reminded if something changes in their campaign. That's why social media is another good way to run a game like this. A lot of players will be more likely to contribute to an online game if they see it whenever they log in to their Facebook account. Setting up some kind of private group or e-mail list is the way to go, as long as everyone is able to view everybody else's posts in a clear and intuitive way.
Next you need to make sure that every post is written in an established format. Use short-hand to differentiate between player discussions and character dialogue. I like to use *asterisks* to identify out of character narration, like this:
*The goblin leaps over the chasm, screaming bloody murder!* "Raaarrrgh!"You will also need to establish a standard method of die rolling for your game. I like to have the DM handle the rolling for the players, since they will know when and where a roll will be required. If the player has to ask "What do I roll for this?" and wait for an answer, it can really slow things down in this format. If you do have players rolling for their own actions, encourage the use of online dice rolling programs to keep things speedy and efficient. I also encourage players and DMs to post the rolls as well as their results to the group whenever they make them. It keeps things transparent and avoids confusion. Use shorthand like:
*Rolled 2d6 +4Not that anyone you would play with would be as silly as to lie or cheat on their rolls, but this helps the group make sure they are all remembering the rules correctly and that things are in proper working order.
You will also need to use some sort of method like a dropbox account, web image host, or web based resources like Mythweavers to keep track of shared documents like character sheets and maps. Make sure everyone knows how to access and refer to these. Also make sure that your players can track their changing stats like experience or hit points. In my games, I have often kept a running tally of these values myself, reposting them from time to time like a changing scorecard just to keep it from getting forgotten.
Make sure that players and DM make their posts as few as possible. Include as much information as you can in each post, both narration, dialogue, and any notes you need to include. The more broken up and numerous the messaging becomes, the longer and more monotonous the game will be. Avoid brief snippets of dialogue like "Who are you?" or "What do you want?" and instead consolidate your interactions as much as possible, saying things like: "I'm Captain Zeke from Star Command, identify yourself and state your intentions or we will open fire!" That kind of line establishes a couple of questions for the player or DM to respond to, as well as an indication of the stakes for the next part of the scene. The more detail in each post, the more progress will be made in the game.
And finally, like I said before, you want to make sure that if a player is taking too long to post that it doesn't mean the end of the game. For this reason, I strongly recommend using non-standard forms of initiative. If you have to wait all day for someone to post while they are at work, it can kill the enjoyment of a climactic battle scene. Let players step in and take their turns whenever they can, and then let the missing player take their own action whenever they want afterwards... or dump initiative altogether if you need to. Keeping a play-by-post game running smoothly is more important than adhering to a specific turn order.
Be ready to adapt to players dropping out or losing interest. PbP games are among the most difficult to maintain interest and attendance. The lack of live, personal interaction can be a real downside for keeping focus. But if the game does flourish and run for a long time, it can be a truly rewarding and enjoyable experience, that lets you put a little extra care and flair into each action as you type it out rather than recite it off the top of your head. It may be different from live games, but it is plenty of fun in its own right, and well worth the effort of coordinating.