A Guide to Play By Post Games.

Basically, this thread exists so that experienced play by posters can impart valuable knowlege to the newbies. Without further ado, here are some suggestions, some of them essential, some of them merely useful, for running and playing in a Play By Post (PBP) game.

Resources

Places to Play (so you can tell them to come here )
myth-weavers.com: One of the best.
plothook.net: One I've used myself and enjoyed.
giantitp.com: Home of the webcomic Order of the Stick, has a large section of its forums devoted to PBP
TheOnline Games section of the Wizards Message Boards have tons of openings if you're willing to wade.
...and many, many more.

Tools:
Online Character Sheets: Several sites including TheTangledWeb, 3edb.com, and my favorite Myth-Weavers offer particularly good sheets.
Eldan recommends rpgwebprofiler.net.

Dice Rollers: WotC has a Dice Roller, as do other sites, but the best by far is over at www.invisiblecastle.com which saves your rolls so you can show the DM. The Planewalker boards also have a built in dice roller, but sites like invisible castle are handy if you want to make several different rolls at once.

Other Resources.

The D20 SRD contains most of the information within the Core rulebooks as well as info on Psionics, the Epic system, and Divine ranks and is absurdly handy for players and DMs (I use it close to daily).

The Official D&D website includes many useful articles, tools, and forums where you can ask for advice. Readers are encouraged to pay close attention to their product excerpts, as they often include cool things like prestige classes that you'd otherwise have to pay for.

And of course we here at planewalker.com like to pride ourselves on being the home of the Planescape setting and one of the best sources of fan-made content on the net. It would be practically criminal not to take advantage of what we have to offer.

General Advice

Know the Game Rules!
I can't tell you how many players, prospective players, and even DMs I've seen get into trouble because they didn't pay enough attention to the rules. I'm not talking about little slip ups like misusing polymorph, I'm talking about big problems like not knowing the difference between a Melee Attack Bonus and a Base Attack Bonus. If you're new to D&D, find someone who can give you the basics or track down a DM who is willing to take new players. Sometimes they can be hard to find, but they're certainly out there.

Know the Setting!
If you want to play in a Campaign Setting (like Planescape), you don't have to be an expert, but you should know enough to not contradict canon every few posts. This goes double for DMs, who really should know better, but we all make mistakes. The key is to come up with a good excuse and pretend you meant for it to be like that all along. It may be dishonest, but it's better than losing the respect of your players. Remember, when you aren't sure about something, look it up. In a PBP game, you really can go off for twenty minutes and refresh yourself on some obscure rule or quirk of the setting.

Listen to the DM!
If the DM issues a new house rule, don't ignore it, follow it! If the DM doesn't want you to use your fancy new new Grey Orc Scout/Barbarian/Monk/Orc Scout build to go 50 miles per hour, don't complain, just make a new character. DMs, on the other hand, should listen their players and make sure they are having fun.

Speak Up.
One of the hardest parts of being in a PBP game is not being able to sit down at the same table as everyone else and talk to them face to face. This means we should spend more time thinking about their thoughts and feelings, not less. It also means that players who are unhappy with the way the game is going need to speak up, as no one else is there to see them scowling. If something is wrong with the game, it is your job to work it out with the DM (preferably privately).

Respect Your Fellow Players!
One problem I have frequently observed in PBP games is what I call the "Meanness Factor." Players in PBP games tend to not know each other, and, therefore, have less knowledge of what is going to annoy another player, and less qualms about employing such tactics. For this reason, Player vs. Player combat, out of character snottiness, and a general disrespect for other players' enjoyment run rampant in many PBP games. No matter how amiable things begin, the Meanness Factor will eventually rear its head in just about any long-running game. The best ways for players to keep these kinds of incidents to a minimum is to always remain polite to your fellow players and keep the lines of communication open. Also, don't even think about starting PvP or anything that could be construed as such until you have spoken with the other player in question. The best way for DMs to keep the Meanness Factor from ruining the game is to have strict no PvP rules (which, to be effective, have to include "borderline" offenses like stealing from or casting spells on another PC). Then, nip any and all PvP that does crop up at the bud before it has a chance to fester into something worse.

Apologize!
If you do insult another player (accidentally or not), break a rule, or very clearly do something you're not supposed to do, apologize immediately. Otherwise you can cause an Incident with a capital "I."

No 4 AM Posts (unless you work nights)!
Don't post when you're exhausted, drunk, or otherwise thinking-impaired. Some of the worst fights I've seen in PBP games occurred when players did something stupid and thoughtless while they were half asleep. If you do make one of these posts, don't argue about how "it made sense at the time" or "it's what my chaotic character would do." Just say you're sorry and move on.

Don't Live In Fear!
In games where there has been a PvP altercation in the past (especially one that basically amounts to bullying) in which one strong PC has beaten the crap out of another PC, there will inevitably be some resentment. Worse, however, is the fear of a repeat incident. A sufficiently cowed PC can wind up feeling like a lame duck and is no fun to play. This kind of PC bullying usually occurs because the players do not respect each other (see above), are competing against each other (see below), and because the DM has been too lax in stopping PvP. There is little a bullied PC can do about this kind of mistreatment besides complaining to the DM about it. If you state your case well, and the DM still does nothing, the game probably isn't worth your time anyway. DMs, on the other hand, have a lot more options in combating this kind of thing. The first thing a DM should do is avoid punishing players for being victims of PvP. Don't charge them for resurrections if they die (and certainly don't make them lose a level), make the other player give back any stuff they've taken, and so on. The second thing you can do is punish the offending player. Don't be overly harsh, but if you make it clear that you don't tolerate PvP and are committed to preventing it, the problem should go away quickly.

Don't Disappear!
PBP games go a bit slower than tabletop games, but that's no excuse to not post for a week at a time in a game where you're supposed to post daily. If the DM expects you to post at a certain rate that you know you can't match, it's usually best to find a different game. If something comes up that will occupy your time for a while, let the DM know so he or she and the other players can plan around it. Otherwise, you might not have a game when you come back!

It's Not a Contest! (If your game actually is a contest, you can ignore this)
One huge hurdle in Play By Post games is that PCs will often start to see each other as competition rather than companions. When this happens, the game in many ways becomes a contest to see who can build the strongest character, get the best items, or even just get the most NPC friends -- in other words, become the biggest Munchkin. At first glance, this usually seems like the players' fault, but in most cases a good portion of the blame lies with the DM. Competition between players can happen for several reasons. The players may not respect each other enough (see above), the Players may be living in fear (also see above), or the the DM has deliberately or accidentally set the players against each other. If it's the second or third case, then it's the DM's fault for putting the players into adversarial roles without thinking about the consequences. Fortunately, there are several things you can do to stop this competition. Firstly, bar PvP, if you haven't already. This will keep the PCs from fighting each other directly and will eliminate much of the point of accumulating power. If that doesn't stop it, put the party back together. Force them into a situation where they need to work together to survive, and watch in awe as enemies become friends again. If you've barred PvP and forced the party to work together, and they are still at each other's throats, then it may be time to consider finding a new game.

You Are Not Your Character!
I can't tell you how many arguments I've seen go something like this:

Paladin: "Dude, how could you kill my character like that? I thought we were friends."
Assassin: "We are friends, but my character is a Chaotic Evil Psychopath and you called him an evil scumbag and turned your back to him, you should have know what would happen!"

Right now you're probably thinking, "Jeez, that Assassin guy sure was a jerk, wasn't he?" Which is true enough, the Assassin (or, more accurately, the assassin's player) was a jerk, but so was the Paladin. Just because your character is Good doesn't mean you get to act all holier than thou and call the evil PCs names (remember what I said about respect), and just because your character is evil doesn't mean you get to go around stabbing people for no good reason (that's just unacceptable). You may be playing characters in a fantasy game, but when you get right down to it, you are still just a bunch of people trying to have a good time. If acting in character precludes you from being a respectful player, then you shouldn't be playing that character in the first place.

Don't Talk too Much!
PC A says something interesting, PC B responds, PC A replies, and the two keep up their discussion of advanced sociophrenology, or Beholder anatomy, or whatever else for a page. Meanwhile PCs D and E's players and the DM are at work or asleep or busy and, despite the fact that all four PCs and however many NPCs are supposedly in the same room, only two seem to be talking. Worse still, the DM was about to advance the plot, but instead of one of the NPCs getting to reveal his dread secret at a moment of tension, all momentum has been lost because the PCs won't shut up about the latest fashions on Ortho, and two of the PCs didn't even get to participate. This is one of those problems that is pretty much unavoidable in Message Board and Email based PBP games. The best thing to do is just accept that extended, natural-sounding conversations are pretty much impossible to do via PBP and just assume that the particularly inane or irrelevant conversations are happening behind the scenes. When it comes to leaving players out of the conversation, it's really not a problem unless the subject is especially relevant to them. That said, if your conversation has gone back and forth between two players for a dozen posts already without anyone else getting involved, it's generally a good idea to take a break and let other people catch up.

Don't Waste Time!
Most PBP happens at a relatively slow rate. One player posts, then you wait a couple hours and another replies, eventually the DM gets home from Nerd Camp and and has an NPC say something and the players respond. It's because of this that even simple actions like ordering a cup of coffee can take a week to finish. Therefore, it's important to makes sure you don't waste time focusing on the small and unimportant stuff (like ordering coffee). When the players decide to go weapon shopping, don't waste a week or two comparison shopping. Just tell them they can buy whatever the want within and parameters you feel like assigning and let the careful haggling happen "off-camera." Other things to skip past include pre-adventure preparation, extraneous chit chat, most travel, and logistical concerns like securing a room at the inn. This applies to PCs as much as the DM. Some DMs are pretty imperious, but in general they won't mind if you skip past something especially pointless without consulting them. It's better than carefully roleplaying having a cup of tea at a restaurant (which I actually saw once, it was even worse than it sounds).