The most important piece of cat furniture should be a good, strong scratching post. Cats need to scratch and cannot be trained not to do it. But the best way to protect your furniture is to provide an adequate alternative.
If the post you have is too small, too wobbly or poorly built, your cat will quickly decide to leave it alone and go back to the couch.
A good scratching post doesn't even have to be a post at all. It could be a board, a box or a cat tree. As long as there is a good surface to stretch up on and exercise the toes and legs, anything you have will do. (This could be why they love the sofa!)
Here are some pointers to consider when choosing or building a scratching device:
1. Shape. If you choose a post, be sure it is tall enough for the cat to reach as far as he can stretch. If you want to "think like a cat," imagine being in the wild, in Nature. Cats scratch on trees, as a rule. They do this to leave claw marks, along with their scent, reaching as high up as they can, presumably to notify others in the area that the cat who made these marks is one big dude, so watch out.
They also scratch as a means of exercising their toes, feet and legs. It's not, as some think, to sharpen their claws. The claws do need to shed an outer layer every so often, so scratching helps the growth process, too. The result may be the appearance of the new, sharper claw, though this is not the sole purpose of the activity.
2. Surface. Probably because it is attractive to humans, most scratching posts are covered with carpeting. However, you will not find any carpet-covered trees outside. Thus, most cats prefer to scratch on bare wood. This is why some cat owners have complained of their cats clawing the woodwork around doors, or the legs of tables. A good scratching post is one made of natural wood, then. Or, a clever do-it-yourselfer may wish to construct one out of two by fours.
If you insist on using carpeting as a scratching surface, don't be upset when kitty wants to scratch on the carpet in the living room.
Another option is to wrap a post with sisal twine or rope. Be prepared to replace the rope every year, because it will get shredded. If you have a supply of carpet samples for this use, try wrapping them around the post or board with the jute side out, and the nice looking carpet on the inside. Again, be prepared to replace these from time to time as well.
3. Construction. Most scratching posts are created vertically. This is fine, but cats at my shelter have always preferred the ones made on a slant. Again, in the wild, cats also enjoy scratching branches, which can be in any position. Slants seem to be popular. However, a "quickie" scratching board I once made was simply a two by six, 3 feet long, with a carpet sample wrapped around and stapled to it, then left on the floor. They enjoy the horizontal scratching also.
The most important part of the construction, though, is strength. If the post is not strong enough to support the weight of at least one cat, it will begin to come apart and will soon be wobbly. Cats will avoid anything that feels unsteady. Also, if a post topples over even once, some cats may never visit it again.
4. Location. If you are trying to keep the cat from destroying a particular piece of furniture, it may work to place the scratching post next to it, to distract them from, say, the sofa, or a table, or even the door jamb. Once they are happily using the new post, try moving it away from the furniture a little bit every day, until it is in another location. Or, if that is not an issue, leave it next to the couch.
One last thing to consider is the cat's claws. It's one thing to use a scratching post, but if kitty likes to stretch up onto your leg while extending his claws, you will do well to keep his claws trimmed. There are also nail caps you can use to cover them, but I have found these to be more trouble than it's worth to apply them.
With close attention to your cat's need to scratch, you don't need to resign yourself to living in a home full of shredded furniture and drapes.