3. Know your Role for what you can roll on - For the most part, it's going to be expected that if you head into a dungeon in a certain role, you're only going to be rolling on loot that fulfills that role. This can be discussed individually in groups (generally it's best to do this before the group actually starts the dungeon), but for the most part, if you're the tank, stick to rolling on tank gear unless no-one in your group needs the DPS gear, and so on and so forth.
Now, there are admittedly a few hitches in this idea in Wrath if only because Blizzard has been working pretty hard to homogenize gear, making it unclear what's the healer gear and what's the damage caster gear and so forth. Knowing your class so that you know exactly which stats are better for you, so you can better judge when to roll and when to pass, can help with this. Of course, sometimes it's not even as clear cut as "This has Spirit, so I should pass," or "This has crit rating, so It'll be best for me," which brings us to our next point.
4. Be Flexible - Understand that there are going to be a few places where the line blurs on what loot is good for what role and what class, and you should be understanding of that blurring. People like getting loot, and sometimes what seems a bit off for them at first glance really isn't.
For example, let's say a cloth caster belt with plenty of spellpower and intellect drops. It turns out that it's a very nice upgrade for 3 people in the part: The Warlock, the Moonkin Druid, and the Paladin healer. Now, each group member may argue that there's reasons the others shouldn't get to roll. The Mage may say that the Paladin and Druid should stick to leather, for example. But in the end, Moonkins and Holy Paladins don't really need the extra armor class on a "higher level" of armor, and a cloth belt may actually be as good for them as any plate or leather belt they could get at the same level. In that case, it's probably for the best that all 3 get to roll on the belt, with the winner taking it graciously, and the losers looking forward to what drops off the next boss.
Likewise, if you're a Fury Warrior and have, say, a Death Knight tank in your group who's usually DPS but is going tank because the group really needs it, consider letting him roll on some of the DPS plate -- and of course, he should let you roll on the tank plate just in case you need to do the same in the future.
5. Be Courteous - This is sort of a companion to the above point. Just remember that there's other people in your group who have different needs and wants as far as loot, and think about them to some extent if you can. If a piece of loot is incredibly good for the Mage in your group and only so-so for you, consider passing. The Mage just may do the same for you on the next boss. If you've already gotten 2 or 3 pieces of really good loot on the run, consider letting someone else have a piece that just dropped. If you're a healer, and a nice cloth spellpower belt with hit rating on it drops, consider letting the DPS have it, since the hit rating is useful for them.
This isn't really something you absolutely have to do, of course, but at the least, getting a good reputation for yourself by being courteous can only help you, and at the least, will make good group members more likely to consider you for an extra group slot in the future.
6. Communicate - This is really the most important of the rules. While the rules up above are all good rules of thumb, and I firmly believe they won't steer you wrong, in the end what you really need to do is communicate. Before the group starts, set everything on the table. Establish the method of rolling for BoP and BoE gear, and establish who will be rolling for what rolls. Finding out that the Tank wants to roll on DPS gear before the instance so you can talk it over is better than a fight erupting midway through the instance because the tank just rolled on a piece of DPS gear.
Communication is what it really boils down to. Find a solution everyone can work with before the instance starts, and as long as everyone follows it cheerfully, you're probably in for a smooth run. Remember, everyone's there for their own reasons, but everyone likes a little loot now and then. Be considerate of others, and they'll be considerate of you. And if you do set up a nice agreeable loot system and don't get the piece you wanted, remember, it may drop for you next time, and if you're lucky, some of these puggies might remember that you were a good sport, and help you out on that run too.