"Always leave people and places better for having known you."
As an aside, having just finished up Fallout 3 (again), on this playthrough I decided to try to be a completely self-serving manipulator when it came to my decisions. Right off the bat I blew up Megaton (first time ever, impressive explosion) because I wanted to get in good with Threepenny and get a nice suite rather than the mud-and-airplane-parts hut I was used to. As I pushed the button on the remote detonator, my karma dropped to 'Very Evil', so I figured, "What the heck!" and killed Threepenny as well. I stole, killed, lied, cheated, and did anything I could to not only survive but to try to better my place in the wasteland.
But funny thing; by the end of the game, just making decisions that I felt would lead to the most beneficial outcome for me personally, I ended up with 'Very Good' karma. I guess on a day-to-day basis, even in-game my good decisions outweigh my bad ones.