Scheduling, Timing, and Pacing
One of the benefits and curses of PbP games is that there is no formal meeting time for games. Players post whenever they are available to check the boards. However, this also extends the time it takes to work through adventures quite a bit. It could take several hours, or even days, for every player to post to a single combat round.
Given this, a few suggestions can make the game run more smoothly. First, state how often you expect players (and yourself, as the GM, in particular) to post at a minimum - once a day, three times a week, whatever works best. If you have particularly prolific posters in your game, especially if the rest of the players don't post as often, establishing a maximum number of posts in a specific time period might be a good idea as well.
Additionally, establish limits for non-responsive players. For example, if a player doesn't post for two days, the GM will decide that PCs actions in order to keep the story moving. Another good thing is to establish a limit to how long a player can be gone (not posting at all), without providing an explanation, before their character is turned into an NPC or otherwise removed from the game. Two weeks is a typical limit.
A Leave of Absence?
The previous paragraph brings up another potential problem of the expanded time span of PbP - player absences. Players might have to leave for a week or more due to vacations, family concerns, work, school, or any of a myriad of reasons. Generally, if the player contacts the GM, that player should be able to return to the game when they get back, without penalty - though, if the absence is going to be especially long, it might be better for the player to leave altogether. Also, its often standard-practice for another player to handle an absent player's character while they are gone. This should only be done with the player's permission, and the GM should make sure the absent player's character doesn't get, well, screwed, while the player is gone.
If you as the GM are going to be away for a while, it’s a good idea to post to both the IC and OOC threads, and let people know when you'll be back.
Dropping and Adding Players
Players end up dropping out of games – whether standard tabletop or PbP – regularly due to outside concerns. This is problematic in PbP due to the extended time span the game progresses through. Ideally, having a few alternate players to call on should a regular drop is a good idea. Otherwise, adding a new character should proceed in the same manner as starting a game.
Rolling Dice and Mechanics
Many GMs prefer to make all rolls themselves, but there are also online dice rollers available. The GM should clearly establish the method used at the beginning of the game and stick to it. If you make only some of the rolls, state which ones players will be making clearly. Also, make sure to provide guidelines on how to report rolls or relevant stats so you can make die rolls. Lastly, explain how you will be reporting die roll results at the beginning to avoid confusion. Players should always follow the guidelines and format suggested by the GM.
The nature of PbP tends to promote heavy RP games. The players have longer to compose their thoughts and responses, and heavy “roll” games tend to go quite slowly, so extra RP helps keep things interesting. Of course, what this means will vary from game to game. If everyone is having fun, then you're at the right degree of RP for your game - whether it's more or less than another PbP is a non-issue. As a GM, you should explain how much RP and detail you expect, and set an example by providing that level of RP and detail in your own posts.
Private Messages and Metagame
Generally, with PbP games, you just have to trust players not to metagame. As a player, you shouldn't take advantage of this trust. However, every once in a while it’s easiest for a GM to send a private message to a player to supply information or ask questions. In this case, I suggest two things: try to stay in character for PMs, make the PM's subject clear that it is in regards to the PbP game, and mention that you've PMed the player in the IC thread.
Maintaining a Game over Play by Post
It isn’t enough to just start your game and expect it to take off. Indeed if that happens, congratulate yourself, because you’ve either written a very compelling campaign, or you’ve happened upon some very dedicated players. It takes work to keep your game flowing and interesting. The fluid and noncommittal nature of forums leads to many games dying out.
First and foremost, keep your players interested. If players lose interest, they’ll stop posting, and if one person stops posting, then everything grinds to a halt. Whenever possible, encourage your players to keep posting, even if it’s just one-liners. Anything is better than nothing. It may be worth making clear at the very beginning that if someone has to stop posting, arrange for a “heads-up” so to speak. Clear communication between all players (and the GM) is a definite must. Some possible ways of rescuing your game might be to set a maximum period of inactivity before the GM may godmode/railroad (or smite) said character to further the game.
So you, or your players aren’t posting, because nobody knows what to do, or because it wouldn’t be in character for someone to speak. First, you need to let your fellow players know, either in character, or in the out of character thread, that you’re still here and participating. At the very least, somebody should be maintaining the “action” whether it be in game or out of game. Feel free to discuss in the out of character thread. As long as everyone is there and checking in, it shouldn’t really matter that you’ve spent the past month trying to solve this one riddle (as long as it’s really good). If your character is stumped, somebody else might have the answer (out of character) that they can just tell you, and things will go on their merry way.
Feedback is a core component of roleplaying in general (not just play by post). As a GM, you should seek feedback and constructive criticism to make sure that everybody is having fun. Other people can’t see your face, and you can’t see theirs. Therefore, it is up to you to find out whether people are legitimately enjoying themselves. If you think something is stupid, say so, but do so nicely, and firmly, lest your thread get derailed.