Posture is important
Meditation is a divine gift. Meditation simplifies our outer life and energises our inner life. Meditation gives us a natural and spontaneous life, a life that becomes so natural and spontaneous that we cannot breathe without being conscious of our own divinity.
When meditating, it is important to keep the spine straight and erect, and to keep the body relaxed. If the body is stiff, the divine and fulfilling qualities that are flowing in and through it during meditation will not be received. The body should not be uncomfortable, either. While you are meditating, your inner being will spontaneously take you to a comfortable position, and then it is up to you to maintain it. The main advantage of the lotus position is that it helps keep the spinal cord straight and erect. But it is not comfortable for most people. So the lotus position is not at all necessary for proper meditation. Many people meditate very well while seated in a chair.
Some people do physical exercises and postures. These exercises, called Hatha Yoga, relax the body and bring peace of mind for a short period. If someone is physically very restless and cannot stay still for more than a second, then these exercises will definitely help. But Hatha Yoga is not at all necessary. There are many aspirants who can just sit and make their minds calm and quiet without doing any Hatha Yoga.
It is not at all advisable to meditate while lying down, even for those who have been meditating for several years. Those who try to meditate while lying down will enter into the world of sleep or into a kind of inner drift or doze. Furthermore, while you are lying down, your breathing is not as satisfactory as it is when you are in a sitting position, since it is not conscious or controlled. Proper breathing is very important in meditation.