Some people find that the use of a mirror virtually doubles the power of their meditation sessions. Sit in front of a mirror and gaze into the reflected image, setting your focus just above the head so that you view the wall behind you. Looking directly at the face or eyes may be too intense an experience for many students, and may lead to silly concern about personal appearance. Using this technique one only views the physical body as a shadowy peripheral silhouette. Continue gazing for twenty minutes, allowing the eyes to deeply relax their focus.
Enjoy the mirror gazing for twenty minutes, then stand for two minutes, maintaining the heightened awareness as you change position. Then resume sitting in quiet meditation for a further twenty minutes with eyes closed. This mirror gazing technique takes forty-two minutes, but may be extended to one full hour if desired, with eyes open and eyes closed sections remaining equal. Please practice this mirror gazing method no more than once a day.
Strong meditation techniques are medicine and you should not overdose.
Mirror gazing is a form of tratak, the ancient yogic practice of fixing one's gaze on an object with total intensity. You may practice tratak by staring at a candle's flame, a distant tree, the picture of an enlightened teacher, or any object that is pleasant to view. While practicing tratak, one must remain motionless and allow oneself to become totally absorbed by the object you are viewing.
SOURCE: http://home.att.net/~meditation/MeditationHandbook.html