Many pet owners find that cats will not scratch at slipcovers, as they do not get enough resistance on looser fabrics.
The best solution is to train your cat from a young age using a simple squirt gun for water aversion therapy coupled with a loud ‘NO’ command.
Aluminum foil pinned over the area where scratching occurs is an effective deterrent, cats don't like the feel on their claws. Eventually they learn to avoid the are and the foil may be removed.
"Sticky Paws" tape to put on the back of your furniture which has a sticky surface that cats do not appreciate. Available at Paws and Fins Shop.
"Feliway" spray containing a modified feline pheromone that sends a coded message to cats telling it that the chair is to be "loved" rather than treated as "territory." Available at Paws and Fins.
"Soft Paws" soft rubber caps that are adhered to the cats claws to render them less harmful. These are available through vets or over the Internet
"Scat Mat" is an electronic zapper that makes the item unpleasant to be on.
Various motion sensors that emit a loud noise when the cat moves in on the furniture in your absence.
Indoor feline "Invisible Fence" that will lightly shock a cat wearing a special collar when the cat crosses over the line of the ‘fence’. These can be installed around specific items of furniture. Available from the Invisible Fence Company.