center page content
This will only work if the DOCTYPE is XHTML Transitional or Strict. IT WILL NOT WORK if your doctype is HTML 4.x. Here is an example of what it should look like:

Put this at the top of your CSS file and you will be good to go!
#html, body {
top: 0px;
right: 0px;
bottom: 0px;
left: 0px;
height: 100%;
margin: auto;