I agree with Jonathan, in the case of a walk-up touch screen UI, a static or animated illustration of a hand touching a soft button is a great introduction. After the intro screen, the subsequent screens should have touchable looking elements in a consistent visual language with the soft button or graphic in the intro screen.
A text label that says "Touch Me" or "Touch to start" seems like an obvious solution unless the product will be used by audiences that speak a different language. This should always be taken into consideration, especially in the case of a public kiosk UI.
If your users are non-techie, or older adults, I wouldn't rely too heavily on what is being done in mobile, touch screen devices. I agree with Andrew that you should study what is being done on ATM screens, Redbox, etc., specifically designed for a broad audience that are in a hurry.
Making the touchable sections of the screen look like real world buttons seems to be an obvious interactive control for both young and old. Thanks Tom for your reference link in my blog: designing for perceived visual affordance. Audiences of a walk up UI usually don't have the time to explore or learn like they might have with an application or even a web site. The kiosk's UI has to be immediately obvious, intuitive and informative. I think you need to imagine a non-reading, 4 or 5 year old trying to use your interface when designing a kiosk UI of this type.