Here are some ideas based on Reynard's guidelines:
Slowed Advancement Chart
Level 1: 0 XP
Level 2: 2,000 XP
Level 3: 5,000 XP
Level 4: 10,000 XP
Level 5: 20,000 XP
Level 6: 37,500 XP
Level 7: 75,000 XP
Level 8: 150,000 XP
Level 9: 250,000 XP
Level 10+: +250,000 XP/level
Give XP for...