Key pose animation allows you to create motion quickly. The principle behind key pose animation is that the transition between the begging pose of a motion and the end pose can create the illusion of movement. For instance, if you draw a person reeling back and ready to sneeze and then draw the same person bent over with gusts of air flying from his face, you will create the illusion of movement when you switch between the two pictures.
This works best when you create exaggerated drawings. If one character is about to punch another character, the first illustration should have the character winding far back. You can then draw the final pose with the fist extended all the way out, the body bent over and the person hit flying backwards.
One way to ensure your animated motion has "oomph" is to add weight to each character's movements. One way to do this is to create a squish in your character on downward motions. For example, when a character places a foot down as part of a step, her entire body can squish slightly before bouncing into the next motion. This will create the illusion of weight on each step. You can exaggerate this for heavy characters. Thinner characters can be squished just a little to give the impression that their steps are light.
Another way to create movement is to do something called "straight ahead" animation. Animators Frank Thomas and Ollie Johnston recommend this style of animation to create flowing movement and high action. This can be shown by drawing a pose then laying down another sheet of paper over the first and directly drawing the next pose completely from scratch. This can cause some warping in the look of the character but will also create a fluid motion.