Find a scratch post made with similar fabric or texture as your couch if you can, spray catnip on it, then place next to your couch. As soon as your cat starts scratching the couch, just move her to the post.
my Marsh loves my couches, and rarely uses the scratching posts... i buy flexible scratchers (ones you hang over the door knob) and with a few tack nails put them on the couch surfaces he uses most, it really works! saves everyone the aggravation - me for all the times i'm angry he's doing it and yelling at him to get away from the couch, and him from avoiding the place he loves to scratch.
I found buying extra scratchers does not work b/c the cat already knows he has a place to sharpen claws and does not adapt to others.