I used to have this same problem. I always try to keep my breathing even now. I find that it's actually easy with practice. I spent some time studying the breathing techniques used in meditation, etc, and apply those to the drum kit. It's very relaxing, and helps me keep my grip loose, etc. I also find that if I ever do tense up, I notice it right away, because my breathing gets off, and I start to feel the effects of that change, so I can calm down, back it off, and get back to a relaxed state. Also, I instantly found I could play and practice for much longer time periods if I was breathing properly.