I'm not sure if this thread will go anywhere, but this is the place to post small tips that people often overlook about PbPs. I'll start--having tried to run three games and tried to play in about five times that number, I know a lot about what not to do. ;D
1. Know the exact rules of your game before posting a Recruitment thread.
2. Post frequently, whether you're the DM or not. One of the #1 killers of Play By Posts is everyone going, "Meh, I'll wait until someone else posts."
3. Be very wary of forming a language, whether it be the verbal components for your spells or just the gnomish tongue. You will be stuck going back to your old posts until the day your character dies, looking up what the mangled Latin is for "Detect Magic".
4. Don't ask your GM for special accommodations. If it's Rise of the Runelords, don't push being a goblin. In general, he'll just slap you and move on to the next applicant. This isn't a rule set in stone, but definitely don't ask twice.
5. Don't ignore the longer Recruitment threads. They aren't necessarily full, especially since sometimes players drop out.
6. Do your best to synch your character with someone else's, even if it's just "we're old war buddies". Not only will it make it way easier to roleplay, the GM is more likely to accept the both of you. In other words, don't be afraid to hang on someone's coattails!
7. Don't join a PbP so you can play an idea you really like. Base the character on the adventure. You are 20% more likely to get accepted.
Agh, that's what I get for only checking one subforum. Thanks, sir.