Before messing with the DOM (document.[...]), it might be smart to wait for the HTML doc to be fully parsed and the DOM tree to be fully built. This can be achieved as follows:

<script type="text/javascript">
function my_dom_manipulator1() { ... }
function my_dom_manipulator2() { ... }
function my_dom_manipulator3() { ... }

// Execute our snippets after the page is parsed.
var old_onload = window.onload;
window.onload = function() {
   if (old_onload != null)
      old_onload();

   my_dom_manipulator1();
   my_dom_manipulator2();
   my_dom_manipulator3();
};
</script>
[download]

For an example of where this makes a difference, refer to Free Nodelet Hack: Nodelets on the left.