head.insertBefore(script, head.firstChild)

http://gihyo.jp/dev/serial/01/crossbrowser-javascript/0008

(function(){
  var script = document.createElement('script');
  var head = document.getElementsByTagName('head')[0];
  script.src = 'xxx.js';
  head.appendChild(script);
})();

(function(){
  var script = document.createElement('script');
  var head = document.getElementsByTagName('head')[0];
  script.src = 'xxx.js';
  head.insertBefore(script, head.firstChild);
})();