To begin a session of sitting meditation, you first need a comfortable seat. You can use any cushion firm enough to support an upright posture. You can also sit in a chair. The main point is to have a relaxed but erect posture, so that your spine is straight. If you're sitting on a cushion, cross your legs comfortably, and if you're sitting on a chair, place your feet evenly and flat on the ground. You can rest your hands in your lap or on your thighs. Your eyes can be half-open with your gaze directed slightly downward a short distance in front of you. The most important point is to have a posture that's both upright and relaxed. The position of your body has a very direct and powerful effect on your mind. An upright posture enables your mind to rest naturally in a calm and peaceful state, and a slouched posture will make it difficult to rest your mind. Once you're sitting comfortably, the main thing is to be fully present. In other words, you have both feet (mentally speaking) inside your state of concentration-not one foot in and one foot out. Your practice is actually easier and more relaxing if you give it your full attention.
-- from Rebel
Buddha: On the Road to Freedom by Dzogchen Ponlop
Rinpoche