Experience points. Every you complete a task, you gain experience points. Gain enough, you go up a level. Flavor them for different job tracks, tweak the rewards to incentivize the behaviors you want and not the ones you don't. If someone isn't gaining experience at a good rate, take a closer look to see whether they are doing things that should be worth XP, or whether you need to have a different conversation.

Give people some choices on leveling up -- maybe most people just want a bump to salary, maybe other people would like to gain more vacation, stock, half days Friday, a private office, a good parking spot, etc etc.