RewriteCond %{REQUEST_URI} ^/$
RewriteCond %{QUERY_STRING} ^s=(.+)$
RewriteRule .* /searchmyblog/%1/? [R,L]

if ( ! function_exists( 't5_search_base' ) )
{
    register_activation_hook(   __FILE__ , 't5_flush_rewrite_on_init' );
    register_deactivation_hook( __FILE__ , 't5_flush_rewrite_on_init' );
    add_action( 'init', 't5_search_base' );

    function t5_search_base()
    {
        $GLOBALS['wp_rewrite']->search_base = 'searchmyblog';
    }

    function t5_flush_rewrite_on_init()
    {
        add_action( 'init', 'flush_rewrite_rules', 11 );
    }
}