how to quit if you get in to trouble?

I've been using XBMC for a while now, I use it in fullscreen. If I ever have a problem, I can never work out how to quit XBMC and return to OSX. I've tried use the force-quit key combo with no success.....I can Alt-Tab, or the OSX equivalent, and see my mouse cursor appear but I can never do anything to actually quit XBMC so inevitably end up powering my mac mini and rebooting....is there a better/easier way?

thanks.