If you are also tired of empty cans filling up your trash, you might be inclined to buy a can compactor (a.k.a Can Crusher). However, if you can't or prefer to do things the old fashioned way (by brute force!), here's a way to compact cans by hand, that doesn't require too much strength.
1. Hold the middle of the can with both hands, so that your forefingers and thumbs form a square around the can.
3. Use your thumbs to bend the aluminum above and under the middle-bend. The right thumb pushes right, and the left one pushes left. Repeat four times for each "corner" of the square from step 2.
4. Place the can on a flat surface, and push on it with your palm, while rotating to the left.
5. The bent-pattern you created causes the can to twist and compress, requiring minimal force. Even a child could compress a can to 1/3 of its full size.
Word of caution - I've never been hurt using this method, but if you press hard enough, you could injure yourself. Also, never attempt to perform step 4 too fast, as it could cause the can to fly to the side or bend in a way to injure your hand. Practice at your own risk!