The following video is a good demonstration of the uppercut technique. Take note of the body movement, the footwork and the short/sharp/controlled movement:
Mike Tyson has probably one of the most devastating uppercuts. The video below is a compilation of his knockout uppercuts.
Unlike the wild uppercuts you might see in movies, the correct technique is a short, sharp movement and should only be delivered when in range or you need to move into range to deliver it correctly. The uppercut is great weapon for getting through a gap in the defence of your opponent between his elbows as Mike Tyson does. Note the twisting of the hips to improve the upward trajectory of the uppercut in order to get through the defence gap.
Also check out the following video that explains the technique in some of Mike’s Hooks and Uppercuts.