In a wonderful lecture Andrew Weil, M.D. noted that breathing is the only thing we can do both consciously and unconsciously. This simple exercise can have profound effects on your overall wellbeing and is cost and time effective.
Sit or stand comfortably.
Tongue to top of mouth.
Let air out through your mouth.
Close mouth and breath in for count of 4 (all counts are ideals but less is fine!).
Hold breath for count of 7.
Exhale through mouth for count of 8.
Repeat 4 cycles (twice daily if you can, start once daily!)