Pro Tip: Get Zoe, assign her as your companion, click on her titties to increase her lust meter, and use Moaning Whispers, (repeat this 4 times). Then when you're fighting enemies you'll get double the experience points and the power lasts for, I want to say, like 5 minutes. When it starts to get low, use it again to restart the time. The power will carry over to any new battle you fight.
This will let you level up in half the time.
EDIT: Also, you get normal XP for fighting enemies the same level, and extra XP for fighting higher-level enemies. Lower-level enemies will get you less XP. Also, consider stripping girls completely for a final XP bonus.