Don't use negative margins. They cause problems.
To center something, all you need to do is define its width as something less than 100% and give it margin:0 auto...
<body>
<div id="centered" style="width:500px; margin:0 auto; border:1px solid black;">
<p>some content</p>
</div>
</body>
The centered div above will be 500 pixels wide and sit centered within the body element.