*attempts to leverage browser caching to raise site speed
*Site speed suddenly jumps to 100
*Very happy for 2 seconds
*Realise it's because I've knocked the site over
*Shit

When caching, build yourself something that automagically appends?version=bla on resources that are referenced within the html, raise the version numbers accordingly.
That way you can opt for aggressive caching and still have a way to force renew on updates etc.
That works because a browser will treat a file with different query parameters as a different file