I think with cats, "training" needs to be mostly from the standpoint of misdirection, positive reinforcement, and choosing your battles wisely. The fact is that you aren't going to be able to go around, telling your cat "do this, not that, this other thing, but not that" and have any degree of success. You need to disabuse yourself of that notion right up front. Silly things like saying "NO!" is only to make you feel better
it doesn't change a cat's behavior one bit. In fact, I swear that sometimes my cats are thinking "hmm, he doesn't like that, so it must be REALLY good". Or perhaps more philsophically: "when he says "NO" it *is* always something REALLY rewarding, so maybe I need to give this a second look next time he has his head turned" (unintended positive reinforcement).
By misdirection I mean that you have to recognize that your cat has needs, like climbing, playing, scratching, drinking, eating, sleeping and so forth. And if the places the cat deems to be the best places to scratch for example are places that you don't want him to. I think you have to:
1. Pick your battles carefully, is the place he picked *so* bad that you can't live with it?
2. What sort of effort have you made to offer what you might reasonably consider to be superior "good" places to scratch? I.e. larger, sturdier, better location, nicer smelling, better texture etc. If you haven't done that, then you need to put more effort into that.
3. Is there a way that you can simply make it impossible for your cat to scratch that item? Move it to a location that is no accessible, or cover it so that it can't be scratched, etc?
4. You can also try to make his favorite place to scratch, less appealing, by moving it to a less desirable (less central) location, or to apply smells that cat's find repellant on it. But if you do this, you have to realize that the need to scratch did not go away, all you are doing is making him select the "next best place" to scratch, which may be an even worse place from your perspective. And of course this may have undesirable side-effects, like if you apply some cat-off to a couch, it may well "cure" his desire to scratch there, but he may also no longer want to curl up in your lap there, or hang out while you watch tv etc.