Clean Permalinks for Search Results
Here is a little tip to force WordPress to use clean permalinks for search results. Despite using pretty permalinks you may have noticed WordPress returns search results as yoursite.com/?s=SEARCHTERMS instead of yoursite.com/search/SEARCHTERMS. I'll show you how to achieve the latter.
First of all, create a new file called search.php which contains the following,
<?php header('Location: http://www.yoursite.com/search/' . $_GET['s']); ?>
Upload that file to root. Next, look in your theme's folder for the searchform. Change the action to,
action="<?php bloginfo('url'); ?>/search.php"
All done. Test it out for yourself. Type the term WordPress in the search engine and look at the URL.
Further reading:
http://codex.wordpress.org/FAQ_Advanced_Topics

