Infinite XP (only without the patch)
You can gain infinite XP if your speech skill is high enough.( about 30 should do). 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. (you sould get about 6XP with each click). Continue to do this and you can rack up XP in mere seconds.
Sandman Glitch (only without the patch)
This a glitch for getting infinite exp Steps: 1.) Reach level 10 with a sneak skill of 60 and get the Mr. Sandman perk. 2.) Go to Andale (south central part of the map) 3.) Wait until around 1am and go into the Smith house while they are sleeping. 4.) Go upstairs into the kid's bedroom 5.) Use sandman on him (crouch and select him and and use the first option) 6.) Repeat step 5 7.) Increase difficulty for more XP!!!! O_o