Our adult cats get their claws trimmed any time we notice them starting to scratch the furniture. When their nails get that long, time to trim!
We were originally using regular nail clippers, just turned sideways. But our boy has such long, thick nails, that the clippers were actually causing the claw to crack. We switched to cat claw clippers, which cost us about $6, look like little scissors, and work great.
Our cats have been having their claws trimmed since they were kittens, so they're used to it and don't move while we're doing it. We've got some stray kittens we're working on at the moment, too.