Note: Using the "Infinite XP" glitches can leave you weak. The higher level you 
are, the more difficult the game gets. You will encounter more and stronger 
enemies, and they will have better weapons in better conditions.
Use the following trick to get 5 experience points each time it is performed.
Go to Craterside Supply (found in Megaton) with a few frag mines (so you do not
have to re-equip the mines). Go into the room behind the mercenary. Get close 
to the back side of the wall that the mercenary is leaning against, and place a
mine. Quickly pick it up as it will immediately be armed and ticking. Repeat 
this as many times as desired.

Have a Speech skill of at least 30. Visit Big Town by travelling northwest from 
Springvale Elementary. Talk to Bittercup about her dating exploits. Talk with 
Pappy about how he came to Big Town with Bittercup to start a Speech challenge. 
If you fail, the challenge will still be available to retry. Repeat the 
challenge until you are successful. You will get six experience points, and the 
option to retry the challenge can still be selected. Repeat the challenge as 
many times as desired. Note: This requires the unpatched version of the game.

Be at least level 10, and have a 60 Sneak skill. Unlock the Mister Sandman perk. 
Go to the Smith's house at Andale. Sneak upstairs at night while everyone is 
sleeping. Try to pickpocket the boy, but instead select "Murder". You will not 
lose Karma and gain over 50 experience points. Because the boy does not actually 
die, you can repeat this as many times as desired. Note: The game may glitch and 
become unresponsive. Also, this requires the unpatched version of the game.