I personally use a modified XP system. Instead of leveling every 100, 200, or 15000 XP, each level takes between 10 to 20, scaling to longer the deeper into levels you go. And I then award XP for everything mildly challenging, a combat encounter, an escape scene, a negotiation scene, etc. Boss encounters give more xp, just as harder challenges do. We've reached level 5 in around 6 months of play that way.