Getting the right contact point with the shuttle on an overhead stroke is extremely important. Often, there are variations in a person's technique that reduces the effectiveness of the the overhead shot. Prepare by turning the body and shoulders perpendicular to the net. Be objective and also get somebody to video yourself - do you really turn enough?
Secondly, many people don't realise they are striking the shuttle with a bent elbow. This means the shuttle is struck at a lower height - the stroke becomes weaker and there is less disguise on the shots when you want to vary it between dropshots, smashes and clears. Think about getting your elbow to the level of your head as you play the overhead stroke and getting the elbow straight as possible when you strike the shuttle.
Thirdly, extra power comes from turning the racquet head from inwards to outwards just as it strikes the shuttle - also known as pronation. To do this well requires finger dexterity so hold the racquet loosely in the fingers until just before the point of impact.