Hi Sarah,
There are lots of things you can do to help your cat. First, here is a great link about it :http://www.vet.cornell.edu/fhc/health_information/brochure_destructive.cfm
Make sure to provide lots of other options for her, scratching posts are great, or the rigid cardboard scratcher boxes–you sprinkle some cat nip on them to lure them and encourage her to scratch.
Clip her nails as short as possible. If you are not comfortable doing this, take her to a groomer or your vet. It’s important to do this every 4 weeks so they don’t get too long. This will help a great deal.
Make sure she has a lot of things to keep her busy,,cat tree, interactive toys, etc..that should help too.
Finally, if you catch her in the act. You can try sound as a deterrent. Take a metal can–like a soup or coffee can, fill it with a few pennies and seal it. When you see her start to try to scratch, loudly shake the can and say NO in a firm voice. After a few times of doing this , you may see her not trying to go near the furniture.
Good Luck! Make sure to check out the link I put in at the top of this, lots of good advice there.
~kelly
It worries me a bit to tell you the truth, can dogs sleepwalk?