- Never, ever, ever tell the players the rules behind the screen.
I believe this is easily the most difficult for a new DM, because the jubilation from a new game can be so high at times we simply want to reveal something, anything, about what's going on behind the screen. But you can't. It's absolutely vital that we don't or the game stops being about the game and now about badgering you. "What would have happened if we opened the chest?" "Is Sjt. Horatio really on our side?" And so on. You can't tell them the answers from behind the screen without destroying everything about the game. So just shore up, keep a poker face, and let them know the players are encouraged to discuss anything with each other, but you can't be a part of it. They're the team, you're the ref.
- Know your rules inside and out.
This goes without saying. If you're going to run a code breaking game, you want to practice sticking to it.
- Prepare only what you need to for each session before each session.
Sessions only last so long, so you only ever need the material the players have a reasonable chance of engaging with for each session's scenario. Don't over prepare, but don't fail to prepare either. Learning what's about right for each group is an ongoing process.
- Let your players play the game, don't play it for them.
This goes along with the first idea. It comes down to not directing your players, but allowing them to direct you. If they aren't taking actions to reveal more about the world, then don't supply that info. Check for clarity, that you're getting your message across, but do not initiate actions for the players/characters.
- Being a fair, trustworthy, and impartial DM doesn't mean you can't root for the players.
You can be objective with the game material, but still want the players to succeed. There's a fashion going around about enjoying your players' losses, but those should be out of your hands. ~edit and up to the inductions of the players to even consider them so.
- Don't be afraid of dead air. Let the players take the initiative unless the timeline says otherwise.
Again this goes with letting the players play, but sometimes that means just sitting there and staring back at them while waiting. This is a great technique as it not only puts them in the driver's seat, but if they don't do anything for long enough, it's inevitably to their detriment as challenges come up in the timeline. Also, sooner or later the casual player will become proactive as they see they have the final word in their character's actions. Afford them the opportunity to do so just as any time in the game when multiple callers are used.