Using a coffee straw, have the student place it in their mouth touching where you would want the tongue to touch (top of the teeth for high brass). Then have them practice blowing constant air through the straw using an "m" shape to imitate what their airstream should be doing. Finally, have them tongue the tip of the straw using "Tu" and listen to it; demo it for them so that they can hear your air stopping and starting by using the tongue to tap the straw.

Eventually, you can wean them off the straw onto the real instrument. Just remind them to think about what it felt like to tap the straw (top of the teeth).