Just a quick idea stolen borrowed from Warhammer FRP 4th edition.
Any player can plead for power from beyond, allowing a reroll on any throw in exchange for taking a corruption point. This can only be done once per roll.
Very evil acts or interaction with corrupted places/items can also cause a gain of corruption points.
Use the corruption rules found in Axioms and reprinted in Heroic handbook.
Lawful divine casters which fall to Neutral due to corruption can no longer cast divine spells. Though the gods of Chaos would be more than happy to take over as their patron. Fallen (Neutral) Clerics can cast divine magic as before, but each use costs 1 futher corruption point (continue to use the spell list for their original lawful patron, the chaos gods are more than happy to play into the characters delusions in order to claim their prize. Clerics which fall to Chaos will then become a Cleric of an appropriate god of chaos instead, from that point using the spell list granted by their new patron.
Optional idea also from WHFRP 4e: allow a player to lose a point of corruption by giving in to temptation: if you happen to fall asleep this watch, you can recover 1 point. etc.
It also might be possible to take on a noble lawful quest to expunge some corruption which has been gained. This should be rare. It may be more likely that such an opportunity be presented to a previous lawful cleric as a chance at redemption depending on their overall goodness of action.
Apply a penalty of 1 to Tampering with Mortality checks for each level of corruption (number of times corruption points have exceeded the character's WIS).