How do I design a comp plan without loopholes?

A comp plan is a machine that runs on human self-interest, and people are very good at finding the shortest path to the money. You are not failing when they find a loophole. You are discovering that you cannot out-think every clever mind at once. Humility here is a design tool, not a weakness. Loopholes live in the gap between what you rewarded and what you actually wanted. You pay for closed deals, so someone closes bad ones. You pay for activity, so someone games activity. The fix is to keep asking what behavior each dollar is really buying, then close the gap between that and the outcome you care about. Take your current plan and, for each incentive, write the worst honest behavior it would reward. Do it coldly, as if you were the rep trying to win. The loopholes you find on paper are the ones you get to fix before they cost you.