Things that make a good PbP game
1. Participation. This is both on behalf of the players and the DM. Lack of participation is the quickest way to kill a PbP game.
1a. Player Participation - When in doubt, post, even if you're not in the immediate scene. Just do an internal monologue, or ponder about something to your familiar, or even your horse! Make some conversation with an NPC or talk to another play that's not in the current scene. Post something even to let the DM know you're still alive and kicking.
1b. DM Participation - When in doubt, post. If your players aren't doing anything, check back a few pages and see if they're waiting for you to respond to something. Sometimes you forget (and players don't feel bad about politely reminding the DM about something he/she may have forgotten), so check back. If the players are dawdling, do the same thing you would do in a face-to-face game, ask "What are you doing next?" Or just drop a random encounter, have some random NPC walk by, or just have them pass by a pretty rock. Post something so they realize they should post as well.
2. For DMs, make the game interesting. It should be something you sincerely want to run, and are willing to run for up to a year or more. Considering the slow pace of PbP, a year may be a minimum investment. Make sure it's something that interests you, and something you feel reasonably comfortable running.
3. For players, make your characters interesting. Loners and those that don't talk much are harder to play in a text medium. Not that all your characters have to be outgoing, but if your guy is the silent type, make sure you put the probmathical "thousand words" into describing his actions. Finding pictures of your characters (Elfwood is good for this, use their Google domain search listed in the linked page) or their equipment makes things much easier to picture for both your fellow players and the DM.
4. Agree on the rules. If the DM indicates he/she's only using certain books, try not to push for material from just "this one other book that I have." If everyone does this, the DM slowly goes insane, and that's a bad thing. I've been guilty of this in the past as a player, but the more I DM, the less I find myself asking for additional books as a player. It can get very confusing, and most DMs only want to check the books that they actually own.
4a. Be careful about making up your own items, particularly in high level or epic games. Trying to deconstruct the item creation rules at high levels causes headaches. Stick with pre-made items, and be conservitive with things that you have custom-made. This will make for less headaches all around.
4b. Just like in face-to-face games, don't argue endlessly over one rule, it puts the other players to sleep, and angers the two that are arguing. Accept the DM's ruling and move on. It's just a game people. Combat will sometimes have to be free and loose because not every DM has a super-duper map-making program. AoOs will not always be taken into account.
4c. DMs, don't be afraid to use Power Word NO! as often as necessary to keep players from running over you. That's a DM's right.
5. Keep with the tone of your game. If the game indicates humor, don't make an excessively morbid character and visa versa.
That's all that comes to mind at the moment. Maybe more later.