Tack a random parameter onto the XML request. For example, if you are using getNetText to download the XML, try something like:
tNetID = getNetText("http://www.server.com/getXML.php?cacheBustingParameter=" & the milliseconds)
Your server will comfortably ignore the parameter passed, but it's enough to fool Director into not using a cached version and downloading the file again.
Instead of waiting for a response you have nothing to lose by trying it for yourself!
But yes, that's the gist of what I was suggesting.