Dzogchen Sky Breath mantra "AH."
Although this is traditionally practiced first thing in the morning, you can also take any moment in the day to wake yourself up and to awaken to the world, or simply to refresh and energize yourself. I find that stopping in the midst of a busy trip to simply take a deep breath and exhale with a great releasing "AH" can perforate the solidity and claustrophobia of an intense day, letting the fresh air of spirit and awareness blow through.
To chant the "AH" mantra, take a deep breath and on your exhalation open your eyes and mouth wide, raise your gaze, and chant a resounding, relieving "Ahhhhh" as far as you can go. Be aware of your out-breath as you chant the mantra and continue and to repeat the mantra from one to three times. You can try to chant at different volumes and in different tones, emptying yourself totally and dissolving with out-breath. As you do this, be aware of how your mind unites and mingle with space and how that space itself mingles with your awareness. Let everything else — thoughts, emotions, feelings, plans, distractions, noises, expectations — dissolve into vast, open space. Simply rest in the view, at ease with everything, including yourself, precisely as you are.
At the end of your exhalation, let the sound of the mantra "AH" resound into nothingness, both within and without, and rest in the utter silence and simplicity of the natural state of just being. When thoughts arise into this state of natural meditation, chant a few lengthy "AHs," following the out-breath. Try to become aware of dissolving a little more with each "AH" repetition. Then, once again, rest in the spacious openness and clarity of natural meditation. You can continue to alternate this dissolving and resting at your own pace, in your own natural way.— Lama Surya Das in Natural Radiance