As a foster parent for cats & kittens; my husband and I have found that using double-sided sticky tape works the absolute best. Once they stick their paws on it, they're deterred. They don't like the sticky feel!
Also, be sure to provide plenty of scratching alternatives: cardboard scratching pads, cat trees, etc. A cat needs to be able to scratch *something*, otherwise they'll destroy your furniture. By providing them with something accessible (and rewarding them for scratching the correct items,) you'll create the good behavior you desire; and the double sided sticky tape will help enforce that.
PetSmart has this: http://www.petsmart.com/product/index.jsp?productI...
You can also just purchase standard double-sided tape, but be aware that it -could- potentially discolor your furniture.
Good luck! :]