Sit with an upright spine, uplift the chest, do not over-arch or slouch in the lower spine, relax the abdomen, relax the shoulders (do not allow the shoulders to roll forward). The chin should be slightly tilted down. Your hands should be folded in your lap or resting on your thighs. Make sure you have enough support under your seat so that your butt is higher than your knees (don't worry if you have to sit on a larger cushion to do this- it is worth it!). This helps create the gentle arch in your low back that is essential for supporting the upper body without creating too much tension in your back. If you are sitting in a cross legged position, you may want also want to put cushions under your knees to support them.