and not to start a pedagogical war. It sounds like you have seen good chop doctors already. If you start overanalyzing the motions of playing too much, you can wind up worse off than you were when you started. This is not automatically to say that people who advocate analysis are bad players or teachers, I am telling you what appears to work for me and my students.
According to a wonderful time spent with Jim Manley who can play from double pedal C to triple C with seemingly little facial strain, I would advocate trying to eliminate tension in the face as much as possible. Think of it like this... You can hold up a pencil with one hand easily. You would not flex the biceps and triceps to full tension for such a small weight - unless you were in the habit of doing so. If you were convinced the pencil was heavy, you would. In this scenario, using all your muscle strength (and/or air power and focus ability) to play would render the high register useless (and you'd probably tire very quickly) since you have already exhausted all your muscle power in the mids and mid high.
Go into a room and turn off the lights. Play a low C (long tone) at a medium soft dynamic and try to stay as relaxed as possible. Listen to the sound! If the sound is resonant (rings the bell even softly), if there is no uncomfortable jerking about, if it sounds "warm" you are there. You may want to experimenting with some pitch bends to help find the most resonant position (or slot) for the low C. It will also be the easiest to play at its most resonant slot. The trumpet is the easiest to play, when played correctly. It is very difficult to play, when you are playing incorrectly.
When you feel you have mastered the C, slur to the D. Find its slot. Then back down to the C.
Continue adding another note each time on top, keeping the feeling of relaxation.
Continue with this exercise, trying to blow each series of notes as if a long tone. Try not to move your chops or alter the air stream. Stay cool, clam (oops calm) and dry... :)
The reason for practicing is so you won't be tempted to look in a mirror, watch tv or be otherwise distracted from listening. This, also, doesn't account for breathing. That is another issue entirely. You've have to have air in the body to get air out. The emptier you are, the harder you must work to get it out.