First thing I do when I get a new system is to redirect downloads from ~/Downloads to /tmp.

The advantage with this adjustment is that in /tmp files older than a week is automatically deleted (and at every restart). Almost all files I download don’t need to be stored, for example:

1. Installers. Run the installer (or dmg) from /tmp and then forget about it and it is automatically deleted within a couple of days.

2. PDFs I just want to read (or possibly print) once.

3. Templates, e.g. expense reports and similar (typically .doc or .xls). I download it, fill it in, generate a pdf and e-mail to the appropriate recipient. No need to keep the original template.

4. Torrents. Download the torrent, add it to your torrent client and then there is no need to keep the original torrent file around anymore. Besides, a lot of the files I download using torrents I just “use” them once so they can be also be downloaded to the same folder as the torrent is stored in, that i …

Source: Mac OSX Hints