Scratching is a normal cat behavior. They do it both for the condition of their nails and as a form of communication. Cats have scent markers in their paw pads and scratching is another way to communicate..."hey, I was here, this is my territory "
In my experience you have to take a multi-pronged approach...you can't really train a cat the way you can with a dog, but you can convince them that there are better places to scratch.
1) get sticky paws ( a double-sided tape) or some other deterant for the areas they've been scratching. Generally cats don't like things that stick to their paws.
2) provide alternatives...get cardboard scratching boxes, a couple of nice tall posts (at least tall enough for a full grown cat to get a good stretch). Some cats like horizontal scratching (ie flat mats or boxes) others like vertical (the post)...still there is a third option (my Stanley) that likes something on an incline.
3) Location, location, location...remember this is a form of communication, so if you put the post hidden away in a corner, it will get ignored. Put these approved scratching surfaces right next to the furnature they were scratching. I also find that my cats like to have something to scratch right next to the door jamb, so we have scratching posts next to the entrances/exits to some rooms.
Hope this helps