Scratching is considered a normal behavior in cats. Scratching things is an inherited trait and cats do it instinctively, but it is also learned from their mother early on in life. Cats scratch in order to leave a visual and olfactory (i.e. smell) territorial marker. Their scent comes from sweat glands located in their paws. To a lesser extent, they also scratch in order to condition and trim their claws, and to provide muscle toning for their front legs, but this is not essential.
Preventing Your Cat from Scratching
To treat destructive scratching, training involves three aspects: prevention, teaching, and correction. Prevention is accomplished by first scratch-proofing your home. This is done by closing-off problem areas and keeping your cat confined to an area that does not have any scratchable objects. If necessary, cover furniture temporarily with plastic and move your curtains out of reach.
Provide a scratching post or board. The post should be of sufficient size and texture to make it appealing to your cat and be sturdy enough to prevent tipping over. The preferred covering should have a longitudinal weave that allows your cat to dig its claws in and get a long stroke. The post should also be taller than your cat when it stands on its hind legs. It must be placed close to where your cat sleeps, since most cats mark their territory near their sleeping areas and prefer to scratch shortly after waking up.
Other hints to minimize scratching include keeping the nails closely trimmed, providing enough play toys, and, if possible, allowing your cat access to the outdoors (under supervision).
Training Your Cat to Use a Scratch Post
You can train your cat to use the scratch post by rubbing the surface with catnip and by rubbing your cat's paws gently on the post (cats prefer previously-scratched objects). If your cat wishes to use an object other that the post for scratching, give him a firm "no", bring him to the post and once again gently rub his paws on the post. You may want to give praise or a treat when the post is used.
Finally, if these steps prove unsuccessful, you can try applying remote correction. This involves hiding out of sight (or using a video camera or mirror around a corner) and applying an unpleasant stimulus whenever your cat scratches the drapes or furniture. Unpleasant stimuli may involve a loud noise, water sprayer, a tin can filled with marbles or a small pillow or stuffed toy thrown near your cat. In this way, your cat will identify the negative stimuli with the act of scratching, rather than with you.