We all know you can open most cheap padlocks with a shim made from an aluminum can. Well, perhaps we didn't all know, but such is the case. However, when I'm standing outside my gym locker after working out and realize I've bonehead forgotten my combination or key, chances are I don't have a premade shim or a pair of scissors, razor blade, marker or a template on me. However, I can probably find a can around the place. Here is a way to make an inferior but working shim using only a can and your hands.
DISCLAIMER: Obviously only do this with your own locks or for others you know have rightful access to what you are opening yet somehow managed to get locked out.
Step 1: Flatten Out the Can
Flatten the can somewhat, either the whole thing or one side. Just squeeze it until it's fairly flat, try not to crumble it too much, we're hoping to get a fairly flat piece of metal out of the deal.
Step 2: Make a Point and Rip
Fold the flat part sharply into a point. Perhaps wiggle it back and forth a few times until the metal starts to give a little, showing through the label. Then flatten it and rip it as one would a piece of paper.
Step 3: Keep Ripping
Keep ripping through the can until it's mostly severed. It will rip easily once the initial tear has been made. If you can, attempt to get one (or more) sides fairly straight as this will be useful later. Be careful, this stuff is pretty sharp, so try not to brush against the edges or corners.
Step 4: Remove Top/bottom
Rip off the top and bottom, leaving you with a nice sheet of aluminum. Toss top/bottom if you'd like.
Step 5: Reduce to Small Piece
Keep doing the same thing you did at first, fold a small point, possibly wiggle it a bit and tear as straight as possible. Reduce it to a piece about half an inch by three inches. It's not terribly picky, but one side needs to be as straight as possible. If it goes bad, pick another sliver from the larger piece and try again.
Step 6: Form Final Shape of Shim
Tear, by the same method, a slit down the center of the piece about half way through it. Fold the outer end in half. Repeat on other side. How long/wide the middle needs to be depends on the lock a bit, it needs to wrap around the piece locking it about half way and need to be deep enough to do into it far enough to unlock it. It's helpful if you can angle both tears a bit away from the center, perhaps even get it a little wavy. Try to deform the center part as little as possible, it needs to stay flat.
Step 7: Use It
Wrap it around the side of the lock that has the hasp below. Pinch it a bit to the side of it, forming it around and flattening it as tight to it as possible. Slide it down, holding the two folded parts. When you feel the catch below move, slide the lock open. Slide the shim itself off and discard.
Step 8: Done
Brief (and rather poorly shot, my apologies) video of me doing this start to finish.