How To Securely Erase Free Space Using Terminal?
From the Terminal app you can use the program srm (secure remove).
Simply start Terminal, then type:

srm &lt;filename&gt;


For more info, take a look at the man page.
Excerpt:


  srm  removes  each  specified file by overwriting, renaming, and truncating it before unlinking. This prevents other people from undeleting or recovering any information about the file from  the command line.