I use to think that stomping on them with my foot, did a pretty good job of flattening the cans, until a friend showed me his version of a can crusher.
He took a piece of 6"x6", cutoff about 10" or so long, and drilled a 3/4 or 7/8" hole down through the center of one end, about halfway. Then he drilled a hole, just the diameter of a marble (1/2 or whatever a marble is), through the side, so that hole intersected the other hole. So now there is an air passageway. Drilled into the center of the other end, about an inch or so, the glued a piece of wood dowel into it, to serve as a grip.
When he lifts that contraption up, and drops it onto a can, the air released from the crushing can, blows up through the bottom hole and out the side. The can is literally 1/3 the thickness of what I can do with my the bottom of my heal.
I said why the marble-size hole? Turns out that spray paint rattle-cans, which he also scraps, have marbles in them. He put a marble in the side hole, lifted the thing straight up, smashed a can, and that marble shot out and hit the wall, like a slingshot. Pretty funny, even if you haven't had a few beers.