The right lane for slower traffic. The left lane is there to allow faster traffic to pass slower traffic.
If you are driving 73 miles per hour when the posted speed limit is 70... you are slower traffic. Stay in the right lane.
If your attempt to pass a vehicle in the left lane takes more than 10-15 seconds, you weren't going fast enough to actually pass them. You are slower traffic. Stay in the right lane.
If you find yourself actually traveling side-by-side with the traffic in the right lane, you had no reason to be in the left lane in the first place, and you're holding up people with places to be. You are slower traffic. Stay in the right lane.
If there is a line of cars trudging along trying to get by in the left lane, and someone (an astute someone who doesn't think he's in a NASCAR race) decides to leave a car length or two between him and the car in front of him, this is not your signal to slide in. Especially when there is nothing but open space behind said vehicle. Stay in the right lane.
If you decide to pull in behind the above astute driver, especially when there is no traffic behind him, remember that there is no traffic behind him, that you can, in fact, wait a few seconds before pulling behind him, and that you will not receive any sort of point bonus for driving up another car's exhaust pipe. You know what to do...
Seriously. I haven't really been on the highway in a few years, but I swear that drivers there have actually gotten worse, which I didn't think was possible. And I didn't even get near the Atlanta area. Yikes...