Includes and paths problem

Is their a proper way to include files so that the paths are always correct. I have a login script as well as protected pages - and the protected pages may be accessed from many possible locations on the site - the included files may also have other included files - so what I have now is a real mess. I'm constantly getting errors saying could not find this file or that file. Is there a way I can make sure these files are available from anywhere. Using an include directory works for some files, but I can't put my whole site in there. Any suggestions are appreciated. C.R.