How to find the Infinite EXP Trick @ Big Town in Fallout 3

Visit Big Town (directly north of Vault 101, on the Vault side of the River) and speak with Bittercup. She will mention dating Pappy. Talk with Pappy and mention Bittercup's remark in a Speech challenge. Even if you fail, the challenge will still be available -- simply try over and over to merit success. Upon success, you'll get 5 EXP, and the option to try again will still remain. Simply repeat successes as long as you like. You can reach level 20 in only a matter of hours, rather than days. Enjoy!
The trick will only work as long as you've gotten on Bittercup's good side (one chance, when you first meet her. If you can't find her, try at night, otherwise she's sleeping somewhere). Pappy must also still be alive, naturally.

Some users have reported that the trick freezes/crashes the PS3, so consider using the "Andale" experience glitch instead.

If you're on a PC, you don't need to do these, you can just type in cheat commands (google them) in the console menu that drops down when you press ~.

Credit:
http://www.gamefaqs.com/