In addition to the mac-specific ways, if you set your Terminal app to Fullscreen (the double diagonal arrows on the top right of Lion+), you can just invoke emacs from the command line. I did this while evaluating whether emacs+evil was something I'd be interested in and it worked just fine.

To be honest, I spend most of my time using OSX in a single fullscreen Terminal.app inside tmux.

I have (set-frame-parameter nil 'fullscreen 'fullboth) in my .emacs, and it just works (GNU Emacs 24.2.91.1 (x86_64-apple-darwin, NS apple-appkit-1038.36)).

If using macports then install using

    port install emacs-app +fullscreen 

Then ns-toggle-fullscreen as mentioned below.

With Emacs 24 you can use ns-toggle-fullscreen.