Pay a bonus to the winning share that finds a block. In a PPLNS payout system this changes the cost of a block withholding attack from 50/50 pool/attacker to >50% cost to the attacker. Scale the size of the payout to your liking.

This is preferable to more heavy handed monitoring schemes because it doesn't compromise privacy or require undue protocol complexity.

Doesn't work for FPPS because FPPS sucks. It is dangerous to all of bitcoin and we must stamp it out.