Drag this link to your browser's toolbar: WayBack.
(If you have problems, try just adding it as a Bookmark/Favorite instead, eg right-click and select "Add to Favorites".)
When you visit a page that you want to find an old version of, just click the toolbar link (or choose the Bookmark/Favorite).
You will be transported to any historic versions stored at the Way Back Machine.
If you're trying to view a page that is now missing (eg, you get a "404" error), this may find you an old version.
[Update: There's an alternative bookmarklet listed under May 16 2002. It might be a bit more robust if you have problems with mine. 15 July 2003.]