Stand with your feet shoulder width apart as this gives you a larger base of support.
Allow the weight to be transmitted though your heels, then find a balance point in the centre of the foot.
Your bottom should be tucked under so that there is not a large curve in the back. There should still be a small curve in the back. The stomach muscles should be drawing in.
Your chin should be tucked in (not up in the air). This is a rotational movement, so as the chin goes in, the back of the head goes up. Think of a balloon attached to the back of the head lifting it up. If you pull the chin straight back, you will get the double chin look (not so good!).
Your shoulders should be relaxed and down, with the bottom of the shoulder blades drawing slightly together. You should feel a lengthening through the front of the body.
From side on, your ear, shoulder, hip and ankle should all be in alignment.
If you are standing in the one place for long periods, like when ironing, place one foot on a raised box or stool in front of you. Alternate the feet to help decrease the strain on the back.
When bending forward over an object (e.g. when bending over the sink to shave, or wash dishes) where possible try to stand with one foot in front of the other. This widens your base of support and helps reduce the strain on the back.