I just like holding them accountable with the tray thing. Yes, it's a bit wasteful, but it's a rare thing; it makes them have to buy new trays eventually if they don't catch it in the trash, or they have to dig it out if they actually do see or find it. I do tell them they messed up in addition to doing that if I have time to, but sometimes there's more of a rush; hence they're fast food places.