html {
       overflow-y: scroll;
}

This is invalid CSS, but it works in everything except Opera. The reason for this is to prevent "centering jumps" when navigating back and forth between pages with enough content to have a vertical scroll bar and pages that do not.

Another solution that works well in all widely used browsers with only a few lines of code, the document height will always be at least one pixel longer.

html, body {
  height: 100%;
  margin: 0 0 1px;
  padding: 0;
}

I typically use them both.  Avoids the ugly bouncing when going from page to page or when AJAX loading a div.