I'll give it a shot:
1.) Don't use too many images and keep them on small size (around 20 kb).
2.) Load as few data from and save to as little data to a SQL database as possible.
3.) Optimize PHP code (especially some heavy and crappy written loops).
4.) Don't use too many CSS sheets, cause each loaded css file is a separate request.
5.) If you have a good webserver, try the increased use of AJAX, to load and reload parts of your page dynamically and don't render the whole content after every click!
Hope this helped