The best way to work with CDR graphics is to of course use the default software for this format - CorelDRAW graphics suite. However, not all users have access to this somewhat expensive software, and have to rely on some 3rd party editing conversion tools.
Conversion to PostScript formats (.ps) is usually done for printing purposes. You can find 3rd party tools like UniConvertor or Inkscape that allows export to PostScript, but as always, using the original software is your best shot because of the compatibility.