Well, it's a cat's natural instinct to scratch and claw. Not only does it help them sharpen their nails, but it's also a way that they mark their territory. If your kitty is old enough to run around and play, then they're old enough to train not to do this.
First off, you have to use a scratchingpost/device that your kitty will find appealing. Cats will rather use the sturdiness and vastness of the corner of your couch than a scratching post that is small and wobbly. So make sure you find a scratching post that is a good size (mine is about 3 feet tall and is part of a kitty condo so it's heavy and sturdy), and be sure to put it in an appealing location. Usually a good place is where the cat is currently scratching. Introduce the kitty to the post. You can even make it more appealing by adding some kitty nip (if there's not some on it since most store bought ones already have it on them). I've found that if you take their front feet and do the "clawing" motion on the post, that they pick it up quicker. I've also found that if you have more than one cat, then you need more than one post. I have four cats, so I have an entire small bedroom that's especially for them. I have two posts, and we've also taken two old nightstands and covered them with carpet. We have carpet all over that room! The cats love it. Since cats are very territorial, you have to give them enough space so that they feel at ease as though it's "their" domain. Otherwise, they'll go around your house trying to scratch up everything to claim what's theirs.
To get them to stop scratching, there's many repellant sprays that you can spray onto the surface that you don't want the kitty scratching up. I've tried some of these, but found that they haven't helped all that much (some ppl swear by them). The best thing that I've found most effective is a water gun or spray bottle. Some may think it's mean, but if you catch the kitty in the act and spritz them once with a little water they'll usually stop.
You can start trimming their nails as soon as they're a few weeks old. It's actually a good idea to do so since their claws can get long enough to scratch up momma and their litter mates. It also helps them to get used to you messing with their feet. Just be sure that when you're trimming, to not go below the quick! The quick is the red vein that you can see in the nail. Usually, clipping the tip is enough but if you accidently cut the quick and the kitty's nail begins to bleed you can put some Quick-Stop (you can get it at pet stores and Wal-Mart) on their nail to make it stop. If your kitty goes outside from time to time, she can still climb to get outta trouble if she has to. Clipping their nails doesn't seem to make it a problem for them.
Finally, if you wanna deter someone from declawing their kitty just tell them that declawing is the equivalent to a human being having the tops of their fingers amputated! It's disgusting, but it's true. Also, if the kitty goes outside (even just once) how can she get out of trouble if she needs to without claws to climb? I've had cats all my life, and I've never had to get one declawed. There's always an alternative.
I hope this helps! :)