But - there is another way, and as usual the PHP core developers are showing us the way. The fastest way to improve percentage of code covered is to REMOVE UNTESTED CODE! Accordingly we saw the test coverage of PHP 5.3 increase from about 55% to a little over 70% over the summer of 2008. At the same time the lines of code in PHP dropped by about 50KLOC. How can this be?

Removing untested code (that's not needed anyway) can reduce unit testing times dramatically and can help you keep your code clean, lean and simple for the upgrades that will come later on.