The best way to grasp several principles of animation is to start animation with a bouncing ball.
Try to animate first a rubber ball, then a Basketball, a bowling ball and a balloon. Try to think about how weight is displayed in animation. Exaggerate more than you are comfortable with, you can always tone it down to a good level afterward.

This is a good exercise to practice timing, squash & stretch, slow in & slow out, arcs and exaggeration.

Hey man, that's a good suggestion.

The bouncing ball IS the cornerstone of so many important animation principles.

Even still I find myself returning to it for help.

Sometimes I reduce my character to a ball in order to block in my animation quickly.

I literally create a sphere of roughly the same size and move that through my scene with the proper weight and timing.

I may also add in boxes for the feet.

This technique allows me to block out my scenes very quickly, and also helps me to get good weight into the animation.

-DJ

one thing I always have found helpful when polishing my animation, is to parent a ball to the controls and see how it is moving for those last finicky little details. As dj said it helps alot to see how your weight is and also helpful for tracking arcs on certain objects that its a little tricky to track. Im just to lazy sometimes to bring out my marker sometimes.
The trick was taught to me by Andrew Gordon when I was in school, for animating and polishing the arms.