My advice:
- Establish up front how often you expect people to post. Establish a time limit for how long you will wait for combat posts before acting for the PC. In practice, acting for a PC sucks because it could lead to a major plot fork - does the PC cast sleep or fireball? Does he sound the retreat?
- Use a short adventure. PBP games take a long time and are liable to peter out before you get very far. OTOH, you can always go for a sequel if things are working well.
- Limit party size to 4 or 5. Having more players can provide the illusion of making things go faster because it's more likely that someone will post something on any given day. It will come back to haunt you when everyone has to post about something, such as in combat or when voting on a path. In the end, games with fewer players tend to get more done per day.
- The DM should make most rolls. That means that a player can roll on invisiblecastle if he wants and post the result, but otherwise you roll. Never ask players to roll something, like a saving throw or spot check, and post the results; you do it. In a face to face game it increases the sense of player participation; in PBP it just slows things down.
- For combat, don't roll initiative for each PC. Roll for the party as a whole, using their best init bonus, and for the NPCs as a whole. When it's the PCs' turn to act, whoever posts first acts first.
- Don't use many random encounters. Every encounter should either be relevant to the plot, or guarding some area, or else "cool" in some way such as making the PCs figure out how to deal with a new monster. If the module had a lot of random encounters, you may need to beef up the fixed encounters to compensate. Even fixed encounters in a module can be removed if they serve no purpose other than wearing down the party's resources; just beef up the interesting encounters.
- Don't give too much importance to any one PC. You never know who is going to leave the game. If you have a 'chosen one' PC and the player leaves, you'll be stuck. Also, other players will be rightly upset if you lavish attention and magic items on one PC. Doing so may be tempting because some people post more than others. Don't get into a long back-and-forth between a PC and an NPC while other PCs are left out.
- Don't use "DMPCs", which are NPCs who travel with the party and outshine the PCs. Players hate that. You might think it's easy to avoid, but that could change when players start dropping out, and you are left with ex-PCs who might be the only ones able to heal or cast fireballs. Get rid of XPCs ASAP, whenever it would make sense in context.
- Some PBP players are jerks. He will post normally one day. Then it's his turn to do something important. Then you wait a few days and there's been no new post from him. That's actually rather normal even for a "daily post" game, so you wait a few more days. You post OOC asking if he's around. Eventually you realize he's not coming back. Expect it. He's not dead; perhaps in a couple of months you'll see him post again, apologizing for not telling you what was up. If you take him back he'll post normally for a few months. Then he's gone AWOL again.
Still willing to hear more if anyone has anything else to add.
