physics my friend.
remember
centre of gravity, (everything pivots around the heaviest point)
some law, one of newtons i think where if you hit something, the same force that goes on them, goes on you. so when you kick something, it actually forces them, AND YOU, away from each other.
move every joint every frame, because when you say lift your leg, its not like you just move your leg, the other leg moves in a bit for balance, your whole body moves slightly, as it will never keep completely still.
the tremble effect is overused, (the shaking of the screen)
its pretty smooth, but these are the things that are making it not look right.