Lately I've been reading about how trombonists (or any other brass instrumentalists) tend to develop tention in their neck or torso when playing in the higher register. In working to decrease mouthpiece pressure, I've noticed that I've started to tense in my throat to control the immense air pressure needed to get the lips to vibrate fast enough. What techniques might I use to feel fast air without resorting to throat tension or mouthpiece pressure? What exercises might I use to preserve that feeling without resorting to bad habits?
I play tuba, and I have learned (and practiced) that when you go up in the registers, firm-up the lips in from the corner of your mouth, not the center. This will reduce the length of the vibrating part of the lips, and like a guitar string, this will naturally generate a higher pitch. It will also make it easier to get a clean sound as you don't strain the air flow at the middle of your lips. When using this technique, you need to be careful not to smile - your mouth should not broaden in a smile, just firm-up at the ends.
I play the trumpet, but this may be relevant to trombone.
You need faster air flow to access to the higher register. A good way to do that is with your tongue. Think of it as whistling: the higher you whistle, the closer your tongue comes to the roof of your mouth. See images there.
Doing that, you restrict the air way. In order not to lose too much air quantity you need to compensate by increasing the pressure before your tongue, with your belly.
So: tongue like when whistling high, then higher air pressure from your lungs.
Try no to press your lips to close together, you'd restrict the air too much. The tongue makes a smaller way, not you lips. Same for your throat. It is hard to restrain from tensing your throat, especially if you're doing weird things with your tongue. Make sure you use support and tongue, and you will have the possibility to relax your throat.
I also have a feeling that the stiffness of the lips matter a lot for playing in the double high register (trumpet), but I'll let someone who can play there talk about this.