Breath Meditation from the Baal Shem Tov

The word for breath in Hebrew is "neshimah," closely related to the word signifying the soul, "neshamah." Thus the breath can become a vehicle for a person to reveal the root of his soul:

As one breathes he should cleave to the Supernal Breath that clings to him too and enters him constantly. As our Sages said on the verse "Every soul (neshamah) utters G-d's praises" (Psalms 150). Thus: "With every breath (neshimah) which a person breathes."

As one exhales and the breath goes out, it returns to its source Above, and then another breath is breathed into one from Above. If a person contemplates the fact that he or she is not breathing at all; rather the Holy Blessed One is breathing in and out of him or her, it will surely be easy for one to attach the part of G-d within them to its root...

(Baal Shem Tov al HaTorah, Noach, Amud HaTefilah 27.)