You can use the header() command to set the cache pragma to no cache.
However, this is not a 100% solution, and there unfortunately is none.
IE on the Mac and Opera do not honor the cache pragma when the page is accessed using the back button of the browser. The no-cache directive is only honored by these browsers when the page is accessed through a link.
All other browsers - to the best of my knowledge - honor the directive even when using the back button.