Depends on how your layout is set, but this is how I usually do it.
CODE
HTML:
<div id="container">
<!-- All my page here -->
</div>
CSS Rule:
div#container{
margin: 0 auto;
width: 800px; // Or however big your content needs to be if it isn't to be fully flexible
}
If its a fluid layout you need, to stretch the whole page, you might have to play with it a bit. I've not made a layout of that kind in a long time, so I don't really remember how exactly, but it would have to do with floats and percentage widths that can get complicated when it comes to cross-browser compatibility (thanks to IE of course...).