For an inline object (text, images, etc.), use CSS to set its containing block's text-align property to center.
For a block-level object (P, DIV, and such) set the left and right margins to auto. The standard defines that when left and right margins are both set to auto, they're made equal (which centers the object). You can do this by manually setting margin-left:auto and margin-right:auto, or you can use the shortand "margin" property if you want.
Unfortunately, the auto-margin trick doesn't work with IE4/Windows. If this doesn't bother you, then fine. Otherwise, put your object in another tag that has text-align:center on it.