One activity that I find very interesting is the making of gear lists. You see people making lists of their ideal gear, and then running instances over and over until they get the exact gear that they want.
This is something that I don't do. Mostly because Lady Chance and I have an understanding: if I want an item, it will not drop.
The only items that I have ever really wanted was the paladin Tier 2, Judgement Armor. I was in a guild that cleared Blackwing Lair. Yet I entered TBC with only one of the eight pieces (the gloves, I believe). I never got my helm to drop from Onyxia. I lost the roll for the legs to a paladin wearing a necklace from Scarlet Monastery.
So I follow a different philosophy: If you do stuff, loot will drop. Do a variety of instances, and upgrade whenever possible. Even if the item is not the "ideal" item for that slot, take it if it is better than your current gear. Spend your DKP freely, and do not hoard it for the perfect item. As long as you keep doing instances, things will work out. Work on all your sets at the same time. If you are Holy, and a good Ret piece drops, spend DKP on it.
Of course, show some courtesy to your fellow players. If something is off-set for you, and main-set for someone else, let them have it.
As long as you keep doing stuff, loot will come. I don't think it's worthwhile to worry about what the "absolute" best piece of gear for the slot.
About the only exception to this philosophy is crafted gear. It's worthwhile to look over the crafted gear in the game, and see if there are any pieces there worth working towards. But this is because crafted gear is not random.
So that's my philosophy. Don't sweat the "ideal" drops, and continuously upgrade. Eventually you'll end up with decent gear.