To convince my family of stuff I often like to show youtube videos, articles, and/or make powerpoint slideshows. Here is a great video about declawing:



A few good articles:

Declaw Surgery
Declawing Cats: Far Worse Than a Manicure : The Humane Society of the United States
What You Need To Know About Declawing - Declawing.com

Hope this helps! Honestly maybe the best thing to point out to them is that cats scratch. It's what they do. If they don't want something that scratches they shouldn't be getting a cat. It's not fair to amputate an animal's toes just to protect inanimate objects from potential damage that can be prevented by other (Much safer) means.