I'm not sure of the best way to learn to do this. Practice, I guess. Draw your extreme poses, make sure they read well, and then draw inbetweens for them. I think the most important thing to do is to make sure that all of the body parts, from the head to the beltline to the hands and feet, should take relatively smooth paths (arcs) - if you traced the location of the hands or feet over the course of the animation, you wouldn't expect them to have very drastic changes of direction or speed over single frames (unless something drastic was happening in that frame, like he hits something with his hands or feet).

This was my first attempt at a run cycle, and the 2 main poses I drew and used for inbetweening later.

While this animation has really poor sense of weight and gravity, I think it at least reads pretty well as a running guy. I should have done tweaks to the up and down motion of his body as he runs at this point, but I didn't. It was a little more painful when I did it later, when I'd drawn more detailed things. If I were to do this animation again, I'd fix the arms, too. The hands take a pretty straight horizontal motion over the animation, and the arm moving forwards and the arm moving backwards take essentially the same path, when you might expect the arm moving forwards to take a higher path than the arm moving backwards.

Here are a few later stages, as I've drawn in more detailed version of the guy on top of the sketch, and extended the animation with the jump. I would recommend doing the entire animation in stick format first, before drawing in any detailed versions... the only reason I didn't here was that I only decided to have him jump after I finished the run cycle.