Iframe integration

I've integrated punBB 1.3 forum into iframe on my site - this works fine, however
I would like also the iframe to dinamically resize only height when navigating trough forum.

Currently I've setup a light javascript:

<script type="text/javascript">
    function resizeFrame(f) {
        alert("tesr");
        f.style.height = f.contentWindow.document.body.scrollHeight + "px";
    }
    resizeFrame(document.getElementById('forum'));
</script>

Which does the job, but unfortunately it is called just once - when forum is loaded.
How could I possibly make it refresh height everytime something inside iframe changes?