This is just an easy way to level up. I didn't discover this cheat, and I don't know who officially did, but this helped me get my last couple levels after I completed the game. I hope it helps others, too. See the video below.
As for reaching rank 30 with Good, Neutral, and Bad Karma, refer to the other solutions. They are very detailed and helpful.
METHOD: Have at least 2 Frag Mines with you and enter the Craterside Supply in Megaton. Throw your Frag Mine against the wall and quickly pick it up over and over again. You will get 6 experience points every time. It is a bit tedious, but it definitely adds up, and is very helpful when there isn't much left to do in the game to earn XP.
If the man leaning against the wall on the other side walks away, just pass time (by pressing the 'back' button) and wait until the next morning and continue. I suggest saving regularly in case something goes wrong.