I second the Irons book, and will add that you should find a teacher to help you if at all possible.

As you ascend, the part of your tongue just behind the tip moves up and forward, causing air to move faster through a tunnel of ever-decreasing size. As far as position goes, to me it feels as if pronouncing "ah-eh-ee-isssss" with the tip of your tongue floating downward behind the front teeth as you progress upward.

There is a critical point to understand, however. The air column is being reduced in diameter between your tongue and the roof of the mouth. You are moving less air volume, but the air is moving at a higher speed. By the time your tongue gets into that hissing position above the staff, you aren't moving all that much volume, but it is moving very fast in a very compact column.

You shouldn't be working much harder above the staff than below. (You will never get more air into the horn than the hole in your mouthpiece allows. The rest just causes back pressure and throat tension). The strongest embouchure around is still a vastly smaller muscle group than the diaphragm. Simply blowing bulk air harder will blow the vibrating aperture open and defeat your goal. It is the combination of a smaller aperture and faster air that raises pitch, not more bulk air volume.

If you can hit a D above the staff, you already have more embouchure strength than you realize. I once had this problem of overcoming embouchure strength with diaphragmatic power and air volume. Don't think "blow more." Think "blow faster" and picture your air column as having the diameter of a pencil lead. Play as pianissimo as you can with a good sound and start climbing scales. See if that makes a difference. Good luck.