Regularity: Set aside the same time or times each day for your meditation. Recommended are dawn (just after awakening), twilight, high noon, and midnight. Another time is in the evening, just before bedtime. It's also easier to meditate on an empty stomach (2-3 hours after meals).
Exercise: If you have time, exercise a little before you meditate. Yoga postures are an excellent way to relax the body and mind before meditation. Paramhansa Yogananda's Energization Exercises are also highly recommended.
Location: Set aside a room, or small part of a room, just for meditation. Try to find as quiet a spot as possible; if this is difficult, try using foam earplugs or headphones to block out the noise. Be sure the room is not stuffy and a little on the cool side; a blanket or shawl to wrap up in is nice.
Sitting: Protect yourself from the downward pull of earth currents by sitting on some natural fiber, like a wool or silk blanket or piece of cloth placed on your meditation chair, bench, or cushion. Sit Erect! Whether you sit on a meditation bench, pillow or on a chair, keep your back straight, chest raised, head erect, eyes closed, and hands resting palms upturned in your lap, preferably at the juncture of the thighs and abdomen.
How long?: Do not set unrealistic goals for yourself. It is better to meditate 5-15 minutes and be very consistent with your practice, and then increase your time as you can. One longer meditation each week can be very helpful. It also helps to meditate with other people, especially with those who have been meditating longer than you. You'll find that group meditations will often help you to meditate longer than you normally would on your own.
Begin your meditation by saying a prayer either out loud or inwardly to God and the Masters, asking them to guide and help you. Do some chanting if you can, using a cassette tape of chants can be helpful. Then practice the breathing exercises to relax: Inhale, tense the whole body, then throw the breath out and relax (do this 2 or 3 times). Then do some measured breathing: inhale, hold, and exhale, then begin again. Do this 6-12 times. Afterwards, relax and breathe normally, and become aware of your breath.
You should feel more relaxed now. Remember to hold the body still. You can mentally check it from time to time to see that no part becomes tense again. Physical tension is a great deterrent to calm and deep meditations. Be very silent and relaxed, yet aware.
Meditate with joy, with devotion. Don't wait for God's joy to make you joyful, be joyful first yourself! Meditation will help you to remember, on ever deepening levels, of who and what you truly are. You are a child of God, and one with the Infinite Light.
Here is a video on how to sit comfortably for meditation: