This is probably the most looked up piece of coding in CSS. Just place the following in your main content or wrapper div.

Code:
margin: 0px auto;
This sets the top and bottom margins to 0px and left and right to auto. The same is achieved with the following:

Code:
margin: 0px auto 0px auto;
Most coders tend to use the first method as opposed to the second, but both achieve the same result.

The Margin tag is followed by Top, Right, Bottom & Left.

Care should always be taken for cross-browser compatibility, especially with Internet Explorer which does not follow standard coding methods.