I should have explained that if you want to center a block level element (like a div etc.) you need to give it a width (which you had) and then set its left and right margins to "auto". So what you've ended up with is:
margin: 10px auto 0 auto
(top, right, bottom, left)
Those autos are for right and left, but the last one (for left) doesn't need to be written, because if left out, it's assumed to be the same as the right margin.