/tmp is your friend

Do you end up with a lot of unnecessary files in your home directory, e.g. in your ~/Downloads folder?  Make it a habit to use /tmp for files that you don't want to keep for a long time.  By default, Ubuntu empties  /tmp every time it boots up so you don't keep accumulating junk.  (If you'd rather keep your temporary files for a longer period, you can change TMPTIME variable in your rcS file.)