As I have written before, brass playing is not about using a lot of power and forcing your lips to vibrate by compressing as much air through your system as possible. It is all about finding the most relaxed way to move a lot of air through your instrument, while keeping your (correct) embouchure in place. These simple exercises should point you in that direction.
The breathing exercises
The two breathing exercises described in the video below are great for brass players. They will help you open up your throat for free air flow, as well as relax your upper body for a more optimal breathing.
The first one described in the video is the Breath Of Fire (go find your inner dragon), and it is a great way to kick start your breathing before playing.
The Root Lock is a tremendeous way to release tensions in your abdominal region. It might make you feel a bit light headed since you force some extra oxygen to your brain, so don´t overdo it. Deep sighs have a similar effect, but I don´t find it quite as effective as the root lock. A good fast fix though.
I would not recommend sitting in the Siddhasana pose (yes, I googled that) while doing the exercises as the cute instructor does in the video unless you are seriously into yoga, or don´t plan ever to walk again.
When you get stressed before a stage perfomance, the common physical indicators are all sort of tensions, increased pulse and forced breathing. All of this is counterproductive to good brass playing. Familiar? Find a calm spot back stage and do these two exercises just before you go on stage. It will help!
Try it out, and please let me know if it does the trick for you. Once or a few times a day just before playing could really make a difference.
PS. If you missed the launch of my new series of books with jazz solos, feel free to check them out at jazzld.com. (Quite a lot of the Digitaltrombone readers picked them up already, so they are either actually really good, or I wrote a killer sales text…)