Don't back off on the air as you get higher. I did that for a long time (still do when I don't pay attention). It can seem like you need to blow "softer" air to get a softer, sweeter, fuller tone up high, but what really happens is you end up not giving the note enough air to speak well. You then bite down on the reed to make a smaller tip opening in order to make the note come out, then it speaks sharp. Dropping your jaw gets a lot easier to do when you're moving more air. Just don't be afraid of the high notes!