Tubists Sam Pilafian and Patrick Sheridan have created a number of resources for wind and brass musicians called “The Breathing Gym.” There is a book and DVD that go together, but this particular 10 minute video I came across on YouTube is a different video than the DVD, but contains many of the same breathing exercises.
One of the great things about these exercises is that they are away from your instrument. Sometimes players will develop a conditioned response to playing their instrument in a way that they automatically breath inefficiently for performing as soon as they put their instrument up to their lips. Practicing breathing exercises away from the instrument can help develop a new breathing habit that is then more easily transferred to performing on the instrument than trying to fight a habit straight out.
Try going through this 10 minute video first thing in the morning just before you practice and see if you notice any difference in your first notes of the day. I’ve found that especially as I get older that in the morning my breathing muscles are a little stiffer and less responsive in general and sometimes will stretch out a bit before I play my first notes of the day. These breathing exercises take this to another level for me and really help me get the air moving.