First, let me say that I'm a certified cat person, but let's face it, we have two cats. As much as I love them, Cats = trouble. How do you keep them from leaving their mark on furniture, walls, door facings, clawing or digging on the indoor plants, or jumping on things they shouldn't (cabinets, tables, etc). We found that a simple spray bottle does the trick. We heard about this from someone else and used it with our first cat, and now that we have a new kitten, we had to start over again.
The trick: Get a small trigger spray bottle (like the one on the left), cheap at Dollar Store, or some other place. Make sure it will spray at least 5 ft. away (don't get one of those tiny spray bottles, on the right) Pour about an inch of white distilled vinegar in the bottom, fill the rest of the bottle with plain water. The cat won't care if it's filtered or not. Each time the cat does something they're not supposed to, spray them. They will quickly learn what they're not supposed to do.
How does it work? Keep the bottle handy. Each time that Sweet Kitty does something he/she is not supposed to, get your bottle (works best if they don't know you are reaching for it, you want to catch them in the act of "evil disobedience" ), take aim and squirt. They don't like the sound or being squirted with water and vinegar. This isn't going to hurt them, and is cheaper than having them declawed. It also helps to have a scratching post, we made ours out of an old post, Sisal rope, and carpet square.
We've even started just making the sound "Chch!" (or whatever sound your bottle makes) of the spray bottle, and they quickly stop. Now, we don't' just do the sound all the time, but vary "Chch!" (or whatever sound your bottle makes) with the actual spraying, that way they don't associate the sound with not getting sprayed. Our 10 yr. old cat still remembers that the sound is associated with being sprayed and stops her misdeeds.
Head on over to Rocks In My Dryer to read more ideas from other bloggers.