What should I do when people stare at me while breastfeeding in public?

Stare Back. It is wonderful that you have no qualms about breast feeding in public. Those who stare usually have issues with it. Staring back can be useful. Decide what you are comfortable doing, and really no one else's opinion really matters.
Stare back? You could slap them, but i suppose that might get you in trouble. How about coldly staring back, and saying "excuse me?".