Sthira Sukham Asanam (Seated posture should be steady and comfortable.)
In the end, what many practitioners find most helpful is simply doing a well-rounded practice consistently. Take the body through its full range of motion-forward bends to lengthen the hamstrings and release the sacrum, backbends and standing poses to strengthen the back and open up the chest and abdominal region, seated poses to open the hips and encourage proper femur rotation, and supine poses to stretch the hamstrings and groins. Not all the preparatory poses need to be active ones; passive poses allow you to open the body without tiring and to draw awareness in toward the Self.
Senior Iyengar teacher Patricia Walden reminds us all to be patient. Most beginning students will find their bodies speaking to them rather loudly when they first sit down to meditate. Richard Rosen suggests that you sit for only a few minutes at a stretch, gradually increasing the time as the posture gets easier. Use props to make your sitting time more comfortable. Sit on folded blankets, a zafu (meditation cushion), or a bolster, so you can keep your knees lower than your hips. You do want to help our knees descend toward the floor, but do not overwork the forward tilt of your pelvis to accomplish this; instead, draw your spine up so your weight comes toward the front edge of your sitting bones. At this same time, allow your tailbone to actively extend toward the floor. As you do this, be careful not to center your weight too far back on your sitting bones; if you do, you will overround your lower back.
If your back spinal muscles insist on sagging, sit against a wall for support. Walden says the wall can give you valuable information. Is one hip closer to the wall than the other? Does one shoulder press into the wall while the other moves away? Use the wall as your guide to gently correct your posture.
If no sitting pose works for you at first, begin with what your body is used to—sit in a chair. But sit mindfully. Richard Rosen suggests sitting so that the top rim of your pelvis (those bony protrusions at the top of your hips) stays parallel to the chair seat and the pubis and tailbone are equidistant from the chair seat. To keep those muscles around the spine active, lift the front body from the perineum (the floor of the pelvis) up through the crown of the head. Rosen says that the front of the spine should feel slightly longer than the back. This length in your spine should help free up the diaphragm, making breathing easier for you. If you have trouble keeping your spine lifted, Walden recommends you sit with your legs through the back of the chair and use your arms on the top of the chairback for support to help you elongate your spine.
Even if you cannot do Padmasana no matter how many different variations of stretching and strengthening poses you practice, you can certainly move toward realizing the benefits of Lotus without falling prey to its difficulties. No matter what sitting pose you choose to use, work toward simultaneously rooting down and lifting up, so that you are aiming to build a sense of groundedness and freedom.