Do a search on this subject as it comes up frequently and has been well addressed.
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mania:Then remember - air you are breathing is at the higher pressure than on the surface so with every breath you take more air than normally. It means you don't need to take deep breaths.
LavaSurfer:..snip..
I found this article. It seems really good so I decided to share it and practice what it teaches and see what happens.
http://www.neptunediveandski.com/breathing.htm
..snip..