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.