Here is a simple breathing exercise that will not only help with breathing efficiency but act as a great core strengthener.
Sit in a comfortable upright position (against the wall if needed), cross legged if possible.
Breath in through your nose into your belly, making your belly big as possible (like Buddha).
Hold for count of one to two.
Exhale all the air through mouth or nose.
When you think you have released all the air, push a little more.
Then, when you really think all the air is released, squeeze your abdominals and hold for a one to two count.
Try this daily, start with sets that last for two minutes, work up to five minute sets.