How to put a website in maintenance mode, for anyone else, except yourself?

Hello everyone,

on wednesday I had the task to upload a major update to one of our companies projects.

My first idea was to disable the system by adding something like the below to the 'index.php' files:

include('include/template/fricking_maintenance.html');
die();

Not such a good idea if you have to test the stuff, because no one, including yourself can access the stuff.
So ... what now? After a short break I got the simple, but stunnig idea. ;-)