I have had good luck with double sided tape. I bought a new couch, and a cat I had staying with us liked scratching it. I put strips of double sided tape across the areas she used (not a solid mass of tape, but strips at intervals across it), and that seemed to work. It also came off the couch with no marks.
I also use the spray that is supposed to deter them. I figured, once the cat uses anything as a scratching post, they will go back to refresh the scent, so you need to get rid of that as well as redirect the behaviour. I spray the deterrent, and I also use the enzyme spray that I bought for the spots where Sam had "accidents" on the carpet.