Don’t overlook the answers posted above already. This thread is resolved.
Evita posted a way to change Autosave in your wp-config file. With that method, you will Not have the problem of losing your changes when you upgrade to a new version of WordPress — a problem which you would have if you actually “hacked the core files” (like wp-settings.php).
Scroll Up to that solution.
Secondly, after that, Exper and Ninjaboy posted plugins. Those also work for this.
Finally, in my experience AutoSave is sometimes quirky when you have the Same Post open on more than 1 computer. It can AutoSave on the computer you aren’t typing on, and then the computer that you ARE typing on will throw up a warning. The feature is somewhat annoying, somewhat useful. It’s very legitimate to wish to disable it, depending on YOUR particular uses of WordPress. Not everyone is sitting there composing the story of their day in a paragraph blog, some people are entering other kinds of data into Posts, that may involve team-editing (collaboration), minute-to-minute updates (live sports blog), etc. To remain flexible in the audience, WP should endeavor to provide options, and in the event that the core doesn’t provide an option, luckily we have workarounds, hacks and plugins to accomplish common needs like this.