Like Steve said: freeze-frame and go picture by picture: it's amazing what you'll find:
for instance, Sometimes a character's body may have growed 200% for a few frames,
and in motion you don't even notice it !
Look in the mirror; walk, jump, run around and then think & count what you just did.
Don't do too may inbetween drawings: dare to keep it on the keys.
Even Disney made that mistake in the CG characters of the Hunchback: the motion was too smooth and interpolated too be 'animation', so they stopped doing everything 24FPS after that.
Oh and when you start drawing: practice practice practice.