I find it a little freaky how much even a brief meditation session affects me. After I'm done, I feel not only calm, but also sort of hyperaware (for me at least) of everything going on around me. Movement, colors, textures, sounds. It's a little like tripping on acid, but without the loopiness and speedy side effects. It also really makes me notice what a chaotic mess my mind normally is.

I'm using the Vajra Guru Mantra, which was suggested in Sogyal Rinpoche's "The Tibetan Book of Living and Dying."

Om Ah Hum Vajra Guru Padma Siddhi Hum

The twelve syllables in the mantra convey the blessings of the twelve teachings of Buddha, which are themselves the distilled essence of the 84,000 Dharmas of the Buddha. So it's really quite a lot of bang for your buck, all the teachings of the Buddha in one tidy little mantra.