Range is the result of Efficiency! While a) it's always difficult to diagnose the exact problem without hearing/seeing you and b) everyone and their mother has range problems, it's generally a rule of thumb that in order to play well in the upper register, EVERY part of our playing needs to be efficient.
One thing that MIGHT help (try it a bit and see if it makes a difference after a few weeks, but don't be married to the idea) is soft playing - but so soft that the lips barely buzz.
Try this warmup/exercise for a bit: Blow into the horn aiming for middle G, and wait patiently for the lips to start buzzing, and play with the softest tone possible. Do middle/low register long tones like this. Then play something like Clarke Studies 1 or 2, starting out just as soft, but crescendoing slightly as the book says to (we lead with the air!).
Think about FOCUSING the aperture, not closing it per say. This approach has definitely helped me with my range - it certainly isn't the golden key to the stratosphere, but it may make things a little clearer (pun intended!).