3rd Register

Keep the source of your sound production low. Involve more lower lip. Everything must come down.

We often do what may seem natural at first - "more air", "blow more" - these are the basic methods we start with as beginners when learning the higher register. This however, places the sound very high in our bodies, creates unnecessary tension in the lips and embouchure, and will be be very hard to do a beautiful and ethereal piano when needed. 

3rd register does require somewhat faster air than the lower register, but always using the minimum for the maximum result. Which means, the slowest air possible to make the sound speak. 

Just like the lighting master Felix Kunze suggests - always doing small changes to achieve great subtle differences.