What is a good posture to use during sitting meditation?

Any posture is okay as long as it is good posture. This means that the spine is balanced on the sit bones (look them up if you don’t know where they are) and retains its natural curve in the lower back. The shoulders are relaxed. The head is well balanced on the spine, with the roof of the mouth parallel to the floor (that’s why you’re told to tuck in your chin).

Leg position can vary — different people have different levels of flexibility — as long as the knees are supported in a stable fashion. Even sitting in a chair is okay.

The eyes are open but cast down, at about a 45 degree angle to the floor.

The hands form an oval, left hand over right, thumbs touching and rounded. They should surround your tanjen (Chinese: tiantien; Japanese: hara), the strong center emphasized in Asian martial arts, located two finger widths below the belly button.

The following link from the Kwan Um School of Zen’s Dharma Mirror illustrates several sitting postures. DharmaMirrorSitting

Why do we walk so formally between sitting periods? Why don’t we just stretch our legs?

Between sitting periods we walk pretty much in unison at a medium or slightly fast pace, keeping the same distance from the person in front of us, with our backs straight and our hands clasped in front of us. Getting up and moving around is very practical. It keeps our legs and backs from getting stiff from sitting still. But we’re not just walking, we’re doing walking meditation. Some styles of walking meditation encourage people to walk slowly and become conscious of every sensation. That is not our style. We walk together, putting aside our personal preferences. Doing this over and over we become skilled at not letting our personal preferences distract us in the rest of our life. We don’t slow down in front of a beautiful piece of art (and there’s a lot of beautiful art in our dharma room) or to read a sign that’s posted. We don’t change our direction because there’s a bird outside we want to look at. We just keep going until we’re signaled to stop. And then we sit down again. In this way we practice not following our desires (to speed up, to slow down, to look at art or birds…). We become attentive to the people around us. Our vision can enlarge beyond our own life.