When playing in the upper register, I often remind myself to whistle out the notes. The tongue placement that we use to whistle certain pitches is somewhat similar to the tongue placement we use to play those same pitches on the trombone. Try whistling a two octave scale. You will notice that as you get higher and higher, the tongue arches more and the air stream begins to move down. I advocate doing something very similar as you go into the upper register of the trombone.