Shizentai and shomenashi

Whenever someone in judo or aikido says the word shizentai it always makes me want to say gesundheit! But that's just me.
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Shizentai is the natural, upright posture used in aikido and judo. The feet are under the hips with slight (but not over-done) separation side-to-side and front-back. Some instructors call this a heel-toe, shoulder-width stance but I think that makes for a little bit too big a stance for shizentai. The stance is generally vertical, with the ears above shoulders above hips above toes. weight is on the balls of the feet (particularly the medial two toes) and is approximately evenly balanced between the feet.
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An important thing to work on is to be able to take one conservative step and recover without getting out of shizentai. This one basic step forms the basis of all footwork in aikido. Try this as an exercise - take one step forward (i.e. shomenashi) and stop halfway through the step just as your moving foot hits the ground. Is your front foot under your center or out in front of you? If it is not under your center then you are stepping too far out of shizentai and you are succeptible to getting immobilized in this wide stance at the bottom of your motion. Now, try the motion like this - from shizentai, shift your center in the direction you're going and then put your front foot directly under your center and freeze. This is a more mobile position (closer to shizentai) and it is not possible to get stuck at the bottom of this step. Any force that is put on you at the bottom of this step just shifts your center out of the way again. It's like your center is floating over your feet.
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Another way of thinking about this same thing is think about always putting feet right under your center as you move instead of positioning your feet in some stance and then trying to get your center back over your feet. Remember - "feet under center - NOT center over feet."