Vertical centering is a bit awkward in CSS unless you are using tables etc.
Here's a routine that works in cases where you know the width and height of the element.
The above will centre horizontally and vertically
<style type="text/css" media="screen">
margin-top: -xxpx; /* where xx = half your elements height */
margin-left: -xxpx; /* where xx= half your elements width */
To just centre horizontally you can use margin-left:auto and margin-right:auto on an element that has a width specified. However you will need to add a text-align:center for ie5 /5.5 to the parent of this element for that browser to centre.
Hope that helps.