The structure of the dining hall was this: There were several tables outside the building, and more tables directly inside, and then there was the entrance to the kitchen/buffet area with the food. On the right side of the entrance, there was a cashier who swiped people's cards as they came in. People would go in, get a tray of food, and come out to eat; later, they might bring their tray back in for more food. Those who went back in just passed by the cashier to the left; you didn't have to pay twice to get seconds.
So, here's what I did: Send in a friend, who picks up two trays at once (still stacked; it looks almost the same as a single tray) with an extra plate (again, stacked under another) and comes out. Out at the table, I take his extra tray, put an extra plate on it, maybe scrub a bit of food on it, put on his lanyard, and carry the tray "back" in to get "seconds". No one ever questioned me.