spin13

I'm going to assume you are practicing zazen since you practice it along with a Japanese art. Though I'll leave a more indepth answer to your own research into Zen philosophy, zazen is the expressed practice of mindfulness.

From a physical perspective, it is not relegated to any particular posture. Lotus, half-lotus, Burmese, and seiza positions are the most common because they are the most stable (as they allow the back to be straight and maintain contact through a tripod position). The exact hand 'mudras' are also of little consequence. What does matter is that whatever position, whatever mudra you decide to use, you use in a very deliberate manner. This means position yourself how you like, but do it exactly as you plan, and be aware of how you are arranging your body each time.

When meditating, you should be aiming only to become aware of what is happening within and without your body. The easiest way to begin is by becoming aware of your breath. You should also be aware of thoughts that arise as well as all the physical sensations that are either with you (breathing, heartbeat, etc) or arise as you sit. The important part is not to judge these feelings, or to dismiss them thinking that they are not a part of meditation. You should embrace each feeling, see how each feeling leads to other thoughts or feelings. Don't follow these thoughts, but accept them, understand where they come from, and release them. You can do the same with each physical sensation as well. It is through accepting each passing and permanent sensation that you can learn how to live mindfully and in the moment.

Become aware, understand the source, embrace and accept, and let go.

-spins