Feedback could be from monitors or the mains. Do you know what system sets it off? If it's the monitors, maybe you need to turn down the stage volume and let the sound reinforcement guy bring it up for the FOH.
If the FOH is triggering it, and you have any flexibility in speaker placement, try moving them forward into the room a bit more.
You may also need to try a feedback destroyer type product. Behringer and peavey make these type units, they are sort of automatic EQs.
Never used one myself.
You could also try the old trick of reversing the phase on the monitor speakers which can sometimes help matters.