My guild did a Round Robin system. Every raider started off as a Raider rank, but a week or two later some people got promoted to Dark Raider if they had shown that they were researching boss tactics, showing up on time, being prepared with potions, contributing to feasts, and showing that they were being active and doing their best during raids.
Raiders could continue to move up to Dark Raider.
At that point then the lists of Dark Raiders and Raiders (split up for tanks, dps, and healers) would be randomized and we'd go from the top. When you got a piece of gear you'd move to the bottom of the list (but healer gear wouldn't just go to the dps or offspec if you were at the bottom of the list.) I can't remember if we allowed passes if the item was an upgrade so you couldn't just gimp yourself waiting for a piece of tier.