(defun delete-trailing-whitespace ()
  "Delete all the trailing whitespace across the current buffer.
All whitespace after the last non-whitespace character in a line is deleted.
This respects narrowing, created by \\[narrow-to-region] and friends.
A formfeed is not considered whitespace by this function."
  (interactive "*")
  (save-match-data
    (save-excursion
      (goto-char (point-min))
      (while (re-search-forward "\\s-$" nil t)
	(skip-syntax-backward "-" (save-excursion (forward-line 0) (point)))
	;; Don't delete formfeeds, even if they are considered whitespace.
	(save-match-data
	  (if (looking-at ".*\f")
	      (goto-char (match-end 0))))
	(delete-region (point) (match-end 0))))))