Ever since I upgraded to 10.3, I have been mysteriously losing hard drive space (this did not happen under 10.2.8). In a typical work day, I will go from 1.1GB of free HD space to less then 300MB, all without adding any files to my system.
I had been resorting to restarting my iBook daily, after which my HD returns to 1.1GB free. I recently discovered that by emptying the trash with the Finder -> Secure Empty Trash menu option, I regain the same amount of space as a restart.
It is only necessary to run the "Secure Empty Trash" option to reclaim disk space, what is in the trash does not matter. Typically I throw away a PDF or create a empty text file to throw away, any size file works.
[robg adds: Testing this hint revealed something else of interest. I had an empty trash can, so I duplicated a PDF and threw it away. Selecting Secure Empty Trash in the Finder, I expected a quick process as the one file was overwritten. Instead, a progress bar appeared, showing 500+ files being securely erased. Based on this experiment, it seems that you can still securely erase files even after emptying them from the trash. I haven't tried to do any low-level work to verify that this is indeed true, but it sure seems like it worked.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031209104734170