A brass players range is one of those skills that frustrates most of us! We work hard to improve and sound like the hero's we've all followed from the time we first heard them!
Most of the information out there is about air, air and more air. Though this IS a vital ingredient to playing, it is NOT the overall answer. When a player has no clue what role the lips play, it's easy to fall into a routine of bad habits that cause frustrating inconsistencies!
Several things that must occur from the lips at once would be...
1) Small focused aperture
2) Relaxed enough to vibrate but supported enough to achieve the note
3) Set close enough to allow air to pass through, but not too far apart to not generate sound.
4) NOT move as the range increases...
By reading "Secrets To Efficient Brass Playing" you'll understand in depth how the lips play a vital role and cannot be overlooked when playing a brass instrument!