You can gain infinite XP if your speech skill is high enough.(My speech was about 30 at the time). Go to town called "Big Town" located north of Vault 101.If you spoke with a girl named Bittercup she should tell you about her dating exploits. After speaking with her, go into the house marked "Common House" and speak to a man named Pappy. There should be a speech skill dialogue option that says "You came here with Bittercup, right?" You will get XP every time you click it. (I got 6XP with each click). Continue to do this and you can rack up XP in mere seconds. Bethesda has not yet patched this glitch.