The biggest barrier to good breath control is physical tension which can spread from any part of your body, causing your chest to tighten, and your shoulders to rise, thus restricting your airway. A few half-decent relaxation exercises go a long way to helping, but the simple act of dropping your shoulders to their natural position and keeping them there throughout the song often does the trick.
The other factor is the dreaded (woman's) head voice, which takes up twice as much air to produce and also can bring about the gear-change (a shift from head to chest voice in the middle of a song - a pain to control, and to listen to). Very few people speak in head voice (Marilyn Monroe maybe), making it basically artificial, so why sing in it?
We had a woman in our workshop who had breathing problems - not surprisingly, as she had a collapsed lung, but she eventually conquered it by moving from head voice to her natural (and very attractive) chest voice.
The older singers, eg Harry Cox and Sam Larner, even in extreme old age, managed to make sense of their songs by breathing in the appropriate places, often by taking snatch breaths with the commas as well as the usual full breaths with the full-stops.
Probably the best example was Phil Tanner; on the very rare occasions his breathing failed him (last line of Banks of Sweet Primroses), it stuck out like a sore thumb because it was so rare.
Jim Carroll