I use the following (from http://www.emacswiki.org/emacs/FullScreen) in my .emacs:
(defun toggle-fullscreen (&optional f)
(interactive)
(let ((current-value (frame-parameter nil 'fullscreen)))
(set-frame-parameter nil 'fullscreen
(if (equal 'fullboth current-value)
(if (boundp 'old-fullscreen) old-fullscreen nil)
(progn (setq old-fullscreen current-value)
'fullboth)))))
(global-set-key [f11] 'toggle-fullscreen)
;; Make new frames fullscreen by default. Note: this hook doesn't do
;; anything to the initial frame if it's in your .emacs, since that file is
;; read _after_ the initial frame is created.
(add-hook 'after-make-frame-functions 'toggle-fullscreen)
Then F11 will toggle fullscreen on/off.