To return clean URIs for search results in WordPress from the search form (www.example.com/search/searchterms instead of www.example.com/?s=searchterms)
First create a file called search.php which contains:
<?php header('Location: http://www.example.com/search/' . $_GET['s']); ?>
Put that file in the root of your WordPress install and alter the action for the search form to this:
action="<?php bloginfo('url'); ?>/search.php"