Add the following lines to your ~/.emacs:

------- BEGIN OF ELISP CODE -------
;(global-set-key (kbd "<f9> s") 'delete-trailing-whitespace)

(defun delete-trailing-whitespace-if-confirmed ()
 "Delete all the trailing whitespace across the current buffer,
asking user for confirmation."
 (if (and (save-excursion (goto-char (point-min))
                          (re-search-forward "[[:space:]]$" nil t))
          (y-or-n-p "Delete trailing whitespace? "))
     (delete-trailing-whitespace)))

(add-hook 'before-save-hook 'delete-trailing-whitespace-if-confirmed)
------- END OF ELISP CODE -------