How do you gain experience or level up in Fallout New Vegas?

yes just turn up the difficulty and you more more exp for everything it said it on the loading screen once.

You can also get the sand man perk by raising your sneak to 60 then travel to andale and make friends with the adults (make sure you dont kill them or the following wont work) then wait for everyone in the town to fall asleep so you can use the sand man perk on the children. (you can use the sand man perk on these children over and over again gaining 65xp each time thus unlimited xp.)