Make sure you have a large scratching post, as Ocimom said. Something with shelves would attract her attention more. You can rub catnip on it. Use this in combination with spraying her with water when you catch her by the couch. Have a bottle of bater near by, and if she starts scratching, try to sprits her with a bit. Most cats hate water, and will quickly stop.
If this doesn't work either purchase special sticky tape or plain old duct tape, attach to her scratching area on the couch sticky side up. She'll get tangled one or twice and stop! And yes I agree with other answers. Trimming claws helps with furniture scratching.
Another approach would be to place her on the scratching post, and gently run her paws along it, showing her what to do. Sometimes scaring a kitty doesn't work (:
I wish you best of luck with that couch!
10 years of caring for cats