There are many .esps to let you better adjust your XP required.
For faster leveling:
XP_Faster_5.esp - decreases XP required for next level by 5%.
XP_Faster_10.esp - decreases XP required for next level by 10%.
XP_Faster_15.esp - decreases XP required for next level by 15%.
XP_Faster_20.esp - decreases XP required for next level by 20%.
XP_Faster_25.esp - decreases XP required for next level by 25%.
XP_Faster_30.esp - decreases XP required for next level by 30%.
XP_Faster_40.esp - decreases XP required for next level by 40%.
XP_Faster_50.esp - decreases XP required for next level by 50%. (it's 2x faster leveling)
XP_Faster_60.esp - decreases XP required for next level by 60%.
XP_Faster_70.esp - decreases XP required for next level by 70%.
XP_Faster_80.esp - decreases XP required for next level by 80%. (it's 5x faster leveling)
XP_Faster_90.esp - decreases XP required for next level by 90%. (it's 10x faster leveling)
XP_Faster_95.esp - decreases XP required for next level by 95%. (it's 20x faster leveling)
And for slower leveling:
XP_Slower_5.esp - increases XP required for next level by 5%.
XP_Slower_10.esp - increases XP required for next level by 10%.
XP_Slower_15.esp - increases XP required for next level by 15%.
XP_Slower_20.esp - increases XP required for next level by 20%.
XP_Slower_25.esp - increases XP required for next level by 25%.
XP_Slower_30.esp - increases XP required for next level by 30%.
XP_Slower_40.esp - increases XP required for next level by 40%.
XP_Slower_50.esp - increases XP required for next level by 50%.
XP_Slower_60.esp - increases XP required for next level by 60%.
XP_Slower_70.esp - increases XP required for next level by 70%.
XP_Slower_80.esp - increases XP required for next level by 80%.
XP_Slower_90.esp - increases XP required for next level by 90%.
XP_Slower_100.esp - increases XP required for next level by 100%. (it's 2x slower leveling)
XP_Slower_125.esp - increases XP required for next level by 125%.
XP_Slower_150.esp - increases XP required for next level by 150%.
XP_Slower_175.esp - increases XP required for next level by 175%.
XP_Slower_200.esp - increases XP required for next level by 200%. (it's 3x slower leveling)
XP_Slower_250.esp - increases XP required for next level by 250%.
XP_Slower_300.esp - increases XP required for next level by 300%. (it's 4x slower leveling)
Optional Plugins:
XP_Slower_Challenges.esp - it reduces XP gained from hack/lockpick/speech challenges to 5-25 from 20-60 (depending on level of the challenge).
XP_Slower_NPCs.esp - it reduces XP gained from killing all NPCs and creatures to 5-35 from 5-50, but I also adjusted levels required to gain XP from them.
You can download this mod here: http://www.newvegasnexus.com/downloads/file.php?id=36411
Also I made version for Fallout 3 if somebody would be interested: http://fallout3nexus.com/downloads/file.php?id=15364
I think I made all necessary plugins to adjust XP, but if you have any requests feel free to ask
