I felt that I had to respectfully reply to this message. Although I am definitely one who can see the danger of overanalysis and getting too far away from actual music making, some breathing devices are very helpful to me as well as my students.
Arnold Jacobs, the great breathing guru and tubist extraordinaire used breathing devices to help students build better breathing habits that will transfer to the trumpet. His breath builder, bags, inspirometer etc. are merely tools to help a student to conceptualize good breathing away from the idiosyncrasies of the horn and to localize the mechanism. Using these devices properly demands that the student think about music more than the technical demands of the trumpet. I would recommend checking out http://www.windsongpress.com/ and read the "Use of Breathing Devices" section to get a better idea. The breath builder and breathing bags are good tools, but the suggestion of a small length of tubing is a good first step.
One exercise that I have my students do without these devices with great success is the following:
Take a difficult passage and blow through it, literally.
1) Put the mouthpiece between your lips and teeth and blow as much air through the horn as you can (REALLY!) while fingering and articulating as normal.
2) Sing the music in your head, but don't try to make high and low sounds as you will be unable to get more than the fundamental sound of each fingering.
3) Breathe whenever you need to and as much as you can while keeping the music going. If this means you inhale while playing some quarter notes, so be it. Keep the fingers going even while inhaling.
4) Listen carefully to make sure that you keep that airflow constant. Don't try to hitch and push for register changes. If you have a low c and a high c right next to each other the sound of the air should remain constant. If you have lip slurs, it should sound like one note not two.
5) Do this a few times and rest when you are feeling dizzy.
6) Play the passage on the horn and try to mimic the air flow you had while blowing.
This is an exercise that gets immediate results. The important things to keep in mind are:
a) Keep the air the same, don't blow harder for high notes and less for low notes.
b) Listen for your articulation. You will find that it is easier to hear unclear articulation this way. Make sure that the tongue is making a complete seal as you articulate.
c) Always have the music running in your head. Hear the music, don't just translate to fingerings and beats.
Try this and then you may understand how to use the breathing devices that are available. The goal is the same. I have heard Vince Penzerella quoted as referring to the breathing bag somewhat like this "Play the bag like the trumpet and the trumpet like the bag" Pardon me if I have misquoted him, but the idea remains.
Good luck to all. Breathe deep and prosper!
Majestic Brass Quintet
Principal Trumpet-Albany Symphony