Breathing techniques can be good to improve focus. To get relaxed enough to pitch can be just as important as having a good bullpen before the game. I find that 5 seconds of inhale through your nose followed by 5 seconds of exhale through the mouth repeated over 1 minute gets me really relaxed. The first few breaths are usually not smooth and flowing, but after a few the breaths settle out. During this process, I just try to clear my mind of any thoughts and focus on the breathing rhythm. After about 1 minute, I'm ready to think about the batters who will be up next inning. I resume normal breathing while I'm planning. After the 3rd out is made, I'm up and at 'em. I follow this same strategy between sets in my bullpens so everything is as close to the game as possible. It helps limit game stress as well.
I find that if I start to notice my breathing on the mound, I'm usually just about out of gas and really have to focus on visualizing the next pitch before I make it. Now I'm really in pitch to contact mode and work the bottom of the zone. It's also hard to keep the ball down when you are tired. Pitching while tired is a downward spiral and should be kept to a minimum.