Less Effort in the High Range
It is important to remember that a muscle bound embouchure will not vibrate easily. Work to shape the aperture and embouchure through flexibility instead of muscularity. Also remember that an aperture that is too closed off in the upper register will result in a high range that sounds pinched and lacks resonance. To work on this, practice your scales very slowly starting in a comfortable register. As you ascend, try to maintain the lack of tension in the embouchure and relative openness of the aperture that you have in the more comfortable ranges.