Fixing the path

Hi. Some time ago I developed a small web application that a bunch of users has installed. I have always used the DOCUMENT_ROOT for my includes, but the other day I installed the application in a subdirectory, and as you've guessed a lot of the includes didn't work. I have been reading up on the subject of absolute vs. relative paths etc. And on the subject of defining your own document root. I need the application to be as user friendly as possible, and I would like to avoid having users enter path details in the configuration. What is the best way to solve this problem? Best regards. Rico.