Stop the Cat scratching the sofa

Since she was a kitten, Buffy the cat has had a liking for scratching our sofa. We've tried loads of stuff from water pistols to siting a real cat scratching post nearby (which the cat has shredded) to try and stop it, and every day when one of us is in the kitchen you hear the familiar noise from the lounge. It's pretty much ruined one arm of the sofa.
So this week we're getting a new sofa and really want to stop it happening to this one. The arms are much lower, so the cat might not want to scratch but looking for ideas to stop the cat scratching the sofa.