This glitch requires a 30 Speech skill level and neutral or friendly with NCR. Go to Camp McCarran, and talk to Sergeant Bitter-Root. You should be able to find him in one of the tents. Ask him about his name, and then there will be an option for a 30 Speech skill level attempt. You can keep saying "Bitter-Root. You have an unusual name.", and each time you say it, you can do the Speech attempt again. Simply say "Bitter-Root. You have an unusual name.", do the Speech attempt, ask to talk about something else, and then repeat the process as many times as desired.
This glitch requires a 30 Speech skill level or the "Confirmed Bachelor" perk and friendly with NCR. Go to HELIOS One, and the woman should automatically talk to you. Once she allows you access to HELIOS One, you should be able to start the "That Lucky Old Sun" mission. Proceed with the mission, and talk to Fantastic and Ignacio Rivas. While talking to Rivas, make sure you get him to reveal that he is a Follower of the Apocalypse by using Speech or Confirmed Bachelor, and agree with his ideals about peace or say you are neutral. Do the mission regularly until you get to the point where you activate the Mainframe Terminal. Configure the Power Grid to "5. Full Region (Emergency Output Level)". Finish the mission by hitting the Reflector Control Panel outside on top of the tower. Go back to Ignacio Rivas, and choose the "I overloaded the plant. No one....." option. That option will never disappear. Keep selecting that option to get 350 XP, 3 Stimpacks, and 2 Doctor's Bags.
If your Speech skill level is 50 or higher, you can persuade Old Ben to offer his escort services to the local bar. You will get 61 XP each time you complete this Speech Challenge. If you follow him back to the bar afterwards and wait until he sits down, you can speak to him again and repeat the same Speech Challenge as many times as desired. You can find Old Ben sitting by a fire close to The King headquarters in Freeside.
This glitch requires a high Speech skill level and 8 Intelligence skill level. Go to the Camp McCarran Terminal building inside Camp McCarran. Talk to the guy at the far end of the building, in the left corner. He is the leader and gives you multiple quests. Accept the interrogation quest, then go meet the woman upstairs to start the quest. When you get in the room to talk to Sirus, pick the "Intelligence" option first, the "Speech" option next, and then the "Intelligence" option. Any of the three "Intelligence" options work. Then, select the "Speech" option (which should now be grayed out), and cycle through the "Intelligence" options. You will get 50 XP every time you cycle through the options.
Go to Vault 11, and you will get a quest to find out what happened there. Once you get the overseer's passcode and enter it into her computer, you will be led to the "sacrificial chamber". Use a Stealthboy to get past everyone, and when the walls on the left open (as you walk in), go into the door and access the computer mainframe. You will be given three options, "Override" and "Download" for two conversations. Select the second download to get 500 XP. Do not exit the mainframe. Just keep selecting that option to get 500 XP each time. Repeat this as many times as desired.
This glitch requires a 35 Barter skill level. Go to Cottonwood Cove (in the southeast corner of the map alongside the river). You will meet a man named Aurelius of Phoenix (he sometimes wanders out of the camp). Speak to him, and choose to barter with his camp for ammunition and supplies. Select this option will get you 35 XP. Keep selecting that option to get 35 XP each time. Repeat this as many times as desired.
This glitch requires a 35 Medicine skill level. Once you gain access to the main strip, find Victor outside the Lucky 38 Casino. He will tell you that Mr. House wants to meet you. Follow him inside, and go up the elevator. Speak with Mr. House (the giant television face that looks like Howard Hughes). Find the dialogue choice pertaining to needing to ask him a few questions. Then, ask "Who exactly are you, Mr. House?", and choose "You appear to be a computer, not a man". Next, select the Medicine skill check response, "The lifespan you're claiming is impossible, except for ghouls and super mutants." You will then get 35 XP (or 39 XP if you have the Swift Learner perk). You can repeat this as many times as desired.
Gamble in a casino, and play roulette. You will get 100 XP no matter if you win or lose.