I've attached a string with bells on it to my door at the back which my munchkin 'rings' when he wants to go outside... super handy. It was REALLY easy to train him to use it too.
And, I can hear it when I'm out of the room. I got the idea out of a book.
Another signal that he used to use when he was a pup, was to stand by the door and swipe his feet on the carpet... (like they do 'after' they poo)..he only does this after he poos now.