I use Hazel (mentioned by drhayes9 too) to monitor various folders and take action regularly. On the delete side, this includes:
* clearing out any items in Downloads > 1week * clearing out any items from my "Temp" folder > 1 day, unless they have an explicit "save" tag ("Temp" is my go-to alternative to the Desktop and is where I stash anything, well, temporary-ish) * automatically moving screenshots into my Temp folder (where they will soon be deleted)