I've got a fairly powerful voice - if you want any proof of that try looking at the complaints about my chorus singing on the thread about the recent Lancaster Maritime Festival !

The main reason that singers (or whistle players) run out of breath or wind is lack of planning. If you are running out of breath in a phrase, think about where you can get some extra breath in, preferably without spoiling the phrasing. Many of the Classical Musical Directors that I have worked with have insisted on singers marking specific breathing points in their scores - and I always do it as a soloist. Planning your breathing first, often gives you a chance to actually improve your phrasing of a song and make it more intelligible. It is perfectly permissible to phrase lines differently in some verses to make up for the fact that some sounds use up more breath.

Melodeon players have even more problems with the use of the air button - they can have too much air !