There are multiple techniques for centering a block element; and the choice for the technique depends on whether you have the size set in percentages or in more absolute values.

Centering an entire page’s contents

body {

  text-align: center;

  min-width: 500px;

}

#wrapper {

  text-align: left;

  width: 500px;

  margin-left: auto;

  margin-right: auto;

}

Commented example

body {

  /* MSIE 5 doesn't center based on auto left/right margins,

     but 'text-align:center' does center top-level divs: */

  text-align: center;

  /* Specify a min-width for the body as wide as the 'wrapper'

     element itself. This prevents negative (i.e. inaccessible)

     left-margins in narrow browser windows when using

     Navigator 6+/Mozilla on Win32: */

  min-width: 500px;

}

#wrapper {

  /* Reset alignment to compensate for ‘text-align:center’: */

  text-align: left;

  /* Specify the width of the element. This should be the same

     as ‘body min-width’: */

  width: 500px;

  /* Set left and right margins to auto, thus centering the

     element in the containing (body) tag: */

  margin-left: auto;

  margin-right: auto;

}

See also
http://www.bluerobot.com/web/css/center1.html

For more about the centering block element, check out
http://css-discuss.incutio.com/?page=CenteringBlockElement