There is no one "great" answer to this question. Usually saving disk space also maximizes performance of your computing platform. We fundamentally recommend:
a) proper maintenance (see other tips below) and
b) the occasional 'spring cleaning' (as also discussed below) combined with the
c) use of a utility to clean your unused space (we like both Eraser or BCWipe).

Want more specifics, well we have found a great PDF file that has some great suggestions in it. You can grab a copy of this file by clicking here: http://www.cellstream.com/free_dl/Maximize.Performance.Windows.XP.pdf