Example of lazy loading asyncronous Javascript file.

/*
Lazy Loading Asyncronous Javascript

Requirements:

- Small. I don't want a big mess for them to include on their sites. 10-15
  lines, tops.

- Stand-alone. The environment is unknown, so we can't rely on any external
  dependencies, like javascript libraries.

- Cross-browser. I have no idea what browsers my customer's customers have, so I
  can't do anything modern or fancy that isn't backwards compatible. I assume at
  least IE6 and up though.

- Asynchronous download. The download of my script should not block the download
  of any script on their sites.

- Lazy Loading. If my site is temporarily slow, I don't want to block the onload
  event from triggering until after our site responds.

- Preserve events. Any events used should not override any events on the
  customer's site. Minimal impact, like I said.

- Don't pollute namespace. Global variables should be avoided, since they could
  conflict with existing javascript.

article: http://friendlybit.com/js/lazy-loading-asyncronous-javascript/

*/

(function () {
    function asyncLoad() {
        var s = document.createElement('script');
        s.type = 'text/javascript';
        s.async = true;
        s.src = 'http://yourdomain.com/script.js';
        var x = document.getElementsByTagName('script')[0];
        x.parentNode.insertBefore(s, x);
    }
    if (window.attachEvent) {
        window.attachEvent('onload', asyncLoad);
    }
    else {
        window.addEventListener('load', asyncLoad, false);
    }
})();