It’s easy! Visit the Internet Archive. Click on Web. Enter the URL you want saved into the “Save Page Now” box. Voila!
For example, I linked in a previous blog post to this article: “Never trust a corporation to do a library’s job.”
But, let’s say, the article vanishes in the fullness of time from that particular URL and you can’t find another URL location for it via Google. You will still be able to see the article at the Internet Archive since I used their “Save Page Now” service.