Every part must move every frame, even if its a minuscule amount.
That is very important ^
make the joints not as prominent [as showing] try to make all parts of the limb flow together but still look separate if that makes sense.
In other words, Don't make your limbs pointy as shit. That's also important ^
Redraw everything every frame and don't use the line tool. Use pencil instead and draw one section of a limb at a time. [Forearm, upper arm, shin, thigh, foot, etc.]
That's not very accurate. But it could help newbies learn and w/e.
Leave some frames for the previous movement to ease into the next one. For example, when you animate a punch, when the arm goes back to "wind up" for it, don't immediately make the arm spring forward after it goes back -- have about 3 frames of the arm just barely moving backwards before moving it forward.
That's a good tip.