In addition to having a stutter, my girlfriend unfortunately has chronic asthma (although the two rarely overlaps she told me) but in order to control her breath to prevent from any flare ups or at least have enough air before she speaks - she would inhale/exhale slow before she speaks or even exercises. Again, with this technique she does this as an asthmatic (not as a stutterer) but I'm sure this can help as well - I don't stutter or have asthma but I learn this technique from here and it does relaxes me and prevent from any breathing issues :)