Purchase the following three skills to increase the amount of XP you can earn:

Headshot XP: Extra XP when killing an enemy with a headshot.
Assassination XP: Extra XP for assassinations.
Tool Kill XP: Extra XP if you kill someone with a tool.

Complete secondary quests (that are not attached to the overall main narrative storyline) to quickly gain experience. While completing secondary quests, try to defeat as many enemies as possible to gain additional XP. Additionally, when selecting a quest from the menu, you can view how much XP you will get for completing it. This allows you to decide if it is worth completing. Extra side quests can also be found at the Informant Table.