This message happens because the page you're trying to load was generated as the result of having submitted POST data. You can only get rid of this error by using GET instead of POST.
The easiest way to convert your scripts to using GET instead of POST is to simply search and replace all instances of $_POST and convert them to $_GET.
One thing to be aware of -- there's a limit to how long your URL can be, so you can't use GET to submit large amounts of data. For example, you wouldn't want to use GET for this forum's reply textarea...