How to keep the header static, always on top while scrolling?

Note: This answer dates from 2010. Consider position: sticky in 2021, as mentioned in another answer.

Use position: fixed on the div that contains your header, with something like

#header {
  position: fixed;
}

#content {
  margin-top: 100px;
}

In this example, when #content starts off 100px below #header, but as the user scrolls, #header stays in place. Of course it goes without saying that you’ll want to make sure #header has a background so that its content will actually be visible when the two divs overlap. Have a look at the position property here: http://reference.sitepoint.com/css/position