Mark's written about winning rigged carny games before, and now the Art of Manliness has a nice little guide to winning five midway challenges, including the secrets behind them (for example, the milk-can has a collar welded around the inside of its mouth that makes it just a few millimeters bigger than the ball you're tossing) and strategies for beating them.
The secret to winning Milk Can is to give the ball a bit of backspin and hit the back of the can's rim. The backspin will decrease the ball's momentum, and instead of bouncing off the can, it will slide into the hole. Easier said than done, of course!
Aim for the back of the rim. Remember, we're trying to deflect the ball in, not sink the ball straight through the hole.
Toss the ball underhanded, but grip the ball on top. This will allow you to give the ball the needed backspin.
Give the ball some backspin as you release it. As you release the ball, give a little flick of the wrist so the ball starts spinning backwards in the air.
Carny season's a few months off, but that gives you time to practice.