I feel like this type of thread shows up every few weeks or so. Anyway, as everyone else has said before, drive right, pass left. The only exceptions are left exits and what not. On 4 lane non-highways, its not quite so simple as there are many more left turns to take, and sometimes the right lane is constantly filled with people coming out of parking lots/etc, and its just safer to cruise in the left lane. But in general, stay right.
Oh, and don't ever pass on the right if you can reasonably pass on the left. Just a couple weeks ago I was in middle of three lanes, and two people come up behind me, one passes on the left, one passes on the right. They then speed up, and merge back into the center lane, almost hitting each other, and then swerve back into their respective lanes. I'm just glad they didn't cause a wreck right in front of me.
DON'T PASS ON THE RIGHT, and if you do, be especially alert when going back into the previous lane, as people aren't expecting you there.
(note: I was justified in my middle lane driving as I had just recently passed someone in the far right lane. The people behind me were not interested in giving me any time to get back into the far right lane)