Why, oh why?!
The first thing you have to do is understand why your feline is determined to wreak havoc on your couch. And no, it's not just to drive you crazy. Cats scratch objects for many reasons, the two main ones being: to exercise their muscles, from their paws to their back (have you noticed how they stretch while they're doing it?) and to mark their territory.
Understanding their reasons will help you be patient and understanding, because even though you can get them to stop scratching, it won't happen overnight. Your kitty has a loving bond with you and wants to make you happy (yes, even if they don't show it too often, they really do like to have your approval), but you have to take it slow if you really want to get rid of this annoying habit for good.