Before the Duel
I. Thou shalt not attempt to duel anyone more than 5 levels below thine own.
Actually, if you really want to be an expert dueler, you shouldn't try to duel anyone below your level, as you've got everything to lose and nothing to gain-- if they beat you, you're a terrible player, and if you beat them, you're just bullying lowbies. But there are exceptions for friends and guildies around here, so 5 levels is a good limit.
II. Thou shalt not send more than one duel request.
Duel spamming is just plain stupid. If someone cancels the duel request, they don't want to duel you-- leave them alone. Sometimes, they'll ask you to challenge again, and in that case it's fine. Or they may challenge you themselves. But if someone cancels the duel on you, don't be a jerk. /chicken isn't necessary-- save that kind of stuff for real PvP. Just move on.
III. Thou shalt wait until the time is appropriate.
Trying to duel someone while they're busy is a no. Attempting a duel while someone is low on health or mana is also a no. Trying to sneak a duel in while someone is grinding or otherwise fighting is a big no. Both sides should be ready, aware, and set when that countdown starts. Anything less is cheating.
During the Duel
IV. Thou shalt not use uncommon buffs or consumables.
Opinions vary on this quite a bit, but as a standard, any buffs or consumables you wouldn't normally use when grinding a mob one-on-one shouldn't be used in a duel. That means no health or mana pots, no raid buffs, no elixirs, and so on. Otherwise, people differ on the rest of the rules-- if you're a cook, should you get to use your food buffs? These commandments say no. But while long cooldown abilities aren't allowed in arenas, they should be allowed in duels (as long as the cooldown is up). Rogue poisons and Shaman elementals are OK, as are Pally bubbles and Warlock healthstones (for Warlocks). Trinkets are OK. Potion injectors are not. Scroll buffs, in general, are not.
V. Thou shalt not allow mobs to interfere with the fight.
If you're being really professional, you can have a spotter watch the fight, and pull away any mobs that happen to join in. Otherwise, you should set the duel in a low mob traffic area, and either pause or restart the duel if mobs show up and start attacking either dueler.
VI. Thou shalt play to the best of your ability.
You'd think this one would be obvious, but see the next commandment. Play your best duel, every time, and you'll have no excuses.
VII. Thou shalt win (and lose) honorably.
If you win, offer a simple /bow or a /cheer and leave it at that. Friendly gloating among guildies is fine, "U r the worst rouge evar" is not. If you lose, accept the result just as honorably, with a /bow or a /nod. Make no excuses-- simply be better next time.
VIII. If thou hast a healing spell, thou shall use it to heal the other player, then thyself.
If you have mana and can heal, heal the other player first, especially if you're the winner. There's no reason not to, and healing the other person will make them more likely to agree to a rematch, or think of you honorably. Honorably duelers happily help their enemies up (only to beat them down again).
IX. Rematches are appreciated, but never guaranteed.
See commandment II as well. If you made a simple mistake during a fight or were distracted in real life, it's perfectly fine to ask for a rematch. But whoever you just dueled has no responsibility to agree to another round. If they refuse, move on and find someone else to duel.
X. Thou shalt have fun.
As usual, this is the last word on everything that happens in this game. Dueling should be a good time, whether you win or lose, because it's a fun way to test your own skills, as a player and as a class, one-on-one against someone else. It's the simplest and easiest form of PvP there is (not to mention that it's one of the best ways to start learning to PvP), and even if you're not a PvPer, a good duel against a respectable player is always a good time.