Scratching is an innate characteristic of cats. They need it to exercise their feet and claws, remove the outer layer of the claws, and mark territory by leaving a visible mark besides their own scent.
Cats' ability to scratch is their natural instinct. Once that is stripped away from them, they may begin to develop an array of strange behaviors. Declawing is a reason why many cats change their behavior due to the excruciating physical pain as well as mental distress caused by the inability to perform a natural task. Many countries have banned declawing due to inhumane reason where declawing is compared to amputation since the declawing operation is equivalent to removing knuckles in humans.
How to teach my cat to scratch the right things?
Cat owners can purchase a scratch post or a scratch pad from the store. Cats will be automatically drawn to scratching those items. By adding a pinch of catnip onto the scratch post, it will stimulate their urge to scratch it as oppose to our beloved furniture. Some cats prefer cardboard over any type of scratch supplies from the store, especially for cats that have a chewing habit. Cardboard boxes are excellent scratching and chewing alternative. In fact scratch pads are made of the same material. Cats like the texture of cardboard which encourages them to flex their claws and stretch their body. Again adding a small amount of catnip will make the cardboard a lot more appealing.
If your cat keeps going back to your favorite furniture and damage continues to be made, you can consider purchasing a furniture saving product called the Soft Paws. They are vinyl caps that fit perfectly around your cats claws without causing any pain or discomfort. They are safe and non-toxic, even if ingested by the cat accidentally, they will not harm the cat's digestive system. Cats can extend and retract their claws in the caps flexibly.
Finally if you keep your cat's nails trimmed, it can reduce your cat's tendency to scratch since scratching is also a natural way for cats to trim their own nails.
Source:
http://www.examiner.com/x-19271-Pets-101-Examiner~y2009m8d4-Cats-101-How-can-I-keep-my-cat-from-scratching