- Don't lie to your players. It's frustrating and people feel like they're being toyed with for your amusement, not theirs.
- Don't take away your player's votes. They feel powerless and lose interest in the game if they don't have a say.
- Don't make your players invincible. Without risk, they lose interest in the game.
- Don't be a player too. You are the host, your goal is to make it fun for the players and let them do their thing. If you want to play too, you shouldn't sign up to host, you should sign up to play.
- Don't create more than one or two new roles. This includes tweaks to existing, established roles. Proper balance takes careful testing, and too many new roles is impossible to balance.
- Don't have more special roles than normal people. Most players should have no ability at all.
- Don't create roles with more than one ability. Remember that a lot of people don't even have one (if you're following commandment #6 as you should).
- Always make sure that the time at which each round will end is known well ahead of time.
- Always have a clear, fair plan on how to resolve ties and abstentions.
- Remember that overpowered abilities spread amongst the sides is not balance, it simply breaks the scale in half.
I've been guilty of breaking some of these before myself, but I learned from them and won't do them again. I hope everyone else is willing to take the pledge before they sign up to host games in the future.