You can waste more energy concentrating on pushing your stomach out, etc than any benefit you will get from the added oxygen.
I found the most efficient way to help breathing is long slow exhales to get all the crap out and in with the fresh stuff. But it's of no real benefit until you're under pressure and starting to breathe heavy.
One other thing to watch out for is during max efforts, don't tense up your upper body and therefore your ability to breath. You will run out of steam real fast. This is when you need to concentrate on breathing the most (long slow exhales).