With Firefox it’s very simple. Just use the following element and the browser will preload your contents.
Contents are preloaded in background, once the whole current page is loaded.

 [html]
<link rel="prefetch" href="/images/nextimage.jpg" />

Problem: only Firefox supports this currently.
Note: a ticket is opened about this in the Chromium project.

For other browsers, use some Javascript triggered by the window.onload event:

 [javascript]
<script type="text/javascript">
window.onload = function() {
    // for images
    var im = new Image();
    im.src = '/images/nextimage.jpg';
    // and for other content
    var req = new XMLHttpRequest();
    req.open('GET', 'nextpage.php', false);
    req.send(null);
};
</script>

Beware of HTTP cache headers sent by the server to the browser.
To preload correctly PHP pages, make your script send an Expires header:

header('Expires: '.gmdate('D, d M Y H:i:s \G\M\T', time() + 3600));

Then, for a simple browser detection from your PHP script:

<?php if (strpos($_SERVER['HTTP_USER_AGENT'], 'Firefox') !== false) { ?>
<link rel="prefetch" href="nextpage.php" />

<?php } else { ?>
<script type="text/javascript">
window.onload = function() {
    var req = new XMLHttpRequest();
    req.open('GET', 'nextpage.php', false);
    req.send(null);
};
</script>
<?php } ?>

Links :

• https://developer.mozilla.org/en/Link_prefetching_FAQ
• http://www.4thkingdom.com/wp-content/uploads/dotclearold/computers/789073-web-site-speed-prefetching-images-css/view-post.html (using jQuery)
• http://orip.org/2009/03/prefetching-javascript-or-anything-with.html (using jQuery)
• commit sur Bizou