You should keep to the right except when passing – especially when you're driving on a two-lane road. If you're on the NJ Turnpike, they even have a sign to remind you of that. But when you're driving on the road, there's an even more important rule to remember.
The Left Lane is the Fast Lane!
The leftmost lane on a road is the fast lane, and as you move towards the right, think of it as medium speed. That means the rightmost lane is the slow lane. Traffic will move more smoothly if we all try to remember this little factoid. The left lane is also considered the passing lane.
If you find that cars are continually passing you on the right to move in front of you, chances are that you are driving slower than the cars around you. If that's the case, please do your part to keep traffic flowing by moving over to the right lane. By not moving to the right, everyone around you has to adjust.
Think about it. If you're driving 55mph in the fast lane then theoretically all the other cars should be driving slower than that. o_O It would take forever to get anywhere.
So please, don't be that guy/girl. If you're not in a rush, stay to the right.
In the same vein, if you're speeding along the highway of life, stay out of the right lane. Roads have speed limits, which means you're not supposed to go over the posted number. If you're in the right lane and a car is driving the speed limit, leave them alone. They're allowed to drive at that speed.
If we all drove with our fellow drivers in mind, maybe there would be less congestion and sudden breaking.