In addition to the good suggestions on feeders here -- don't pick up after them.
Let the feeder go empty, and they WILL eat what they've billed onto the floor.
My girls are in the Clean Plate Club, because about once a week I let the feeder stand empty for several hours. They have access to acres of pasture, so they will not starve.
Since I use a deep litter system, I'm not "cleaning out" the coop and discarding their food. I seldom find any feed in the litter when I rake.
If the feed tray fills up with fines -- which are hard for the chickens to pick up -- I just dump 'em in a dog bowl, add water, and give them a wet mash. They act like it's this great treat I'm giving them, and polish it off in minutes.