Really, your best bet is to pick up some reference books such as Dynamic Figure Drawing by Burne Hogarth or How to Draw vol. 1 by Wizard Magazine, and really concentrate on mastering anatomy and figure drawing. Doing alot of life drawing and gesture drawing (quick sketches of poses captured from live reference) will be very helpful as well.
Of course, since manga characters aren't built with photo realistic anatomy (ex. bigger heads, smaller torsos, slimmer abdomen) you'll want to adjust the lessons accordingly. For manga bodies I usually use the value 1 complete body = 5 1/2 heads (normal human height is 1 body = 7 heads)
It'll take a while to master and it's a lot of hard work, but when you get the structure of the body down your art will be all the stronger for it.
You can also try doing character turnarounds, though these are
really annoying and boring to draw. If you want to make sure everything lines up as planned, a turnaround acts like a character blueprint.