Forearm Technique:
OK, by now you've probably developed your own style of hitting the shuttle, probably whatever you've found works for you. Now it's time to change your style. Most likely, if you came from the old school of training, you were told to use your wrist. Unfortunately that is not the right way to do it .
Hold the racket in the forehand position, with the correct hand grip.

Pull the racket back in the direction of the arrow, turning your forearm as you do so. You should now be able to see the underside of your forearm. You should also be able to feel the side muscles in your forearm tightening. Quickly swing the racket in the opposite direction. This is the primary action you should use.

When using the forearm-technique for overhead shots, you should lead with your elbow (ie. as you turn your body to hit the shot, your racket-elbow comes up, followed by your forearm turn). Your arm will swing forward and your body will follow-through for maximum power transfer.

Forehand:
Hopefully you've got the forearm technique going. Eventually, you should be able to clear the shuttle from one end of the court to the other without swinging your arm - might sound impossible, but you can get a lot of power out of your forearm. To hit it harder, you need to get behind the shuttle, and move your body hard into the shot. Tennis players demonstrate it well when they serve. Analysing the physics of the game, to hit the shuttle further and more quickly you need to get the racket head moving as fast as possible. Regard the racket as being part of your arm. With your shoulder as the pivot point, the point of contact (racket head) has a fairly long extension from the pivot point. Thus, by moving your shoulder faster the racket head will move a magnitude of times faster (the longer your arms, the better!).
You should be able to hit the shuttle from one end of the court to the other (and not just with a fast shuttle!). You might like to try hitting it from a metre of so further back - the added power will be invaluable when you're under pressure, or playing with a particularly slow shuttle, or on a cold day.

Backhand:

The best way to play the backhand, is not to play it at all. Use your speed to play overhead shots ‘around the head', that is, to move as far to the backhand side of the court and play a forehand shot. It'll help you to avoid getting into trouble if you can't clear with your
backhand. Bend your back!
If you can't get around (either because it is practically impossible or undesirable), the technique to use is identical to the forearm technique, only in reverse. Most people have no problems executing this on shots at shoulder level or lower, but many have difficulty when the shot is above their head.
To gain extra power in your backhand shots, first get into position by turning your back to the net. Then spin around, hitting the shuttle flat down the line. Always use the weight of your body to maximise power transfer.

Source: TheIndiaSun