I've just spent the last two weeks doing a Xero tour of the UK where we presented five sessions a day in short order. During one session it occurred to me as I stood at the back of the room trying to gesture to the presenter that they had ten minutes remaining before the break, that there ought to be an app for that.
I'm such a genius that I built one in five minutes and I called it Presenter Pitboard. It's open source, runs on any platform and I'm gifting it to the world.
I designed the UI so that an idiot could use it. Just load up this PDF into your tablet or smartphone device, select the message you'd like to discreetly convey to your errant presenter, hold it aloft and, BAM! - presentation zen is yours.