search form – autoclear on 2010 theme

Hi,

I am using the 2010 theme – where the search form is located in /wp-includes/general-template.php.

First of all – how do i make this so the custom code is not overwritten in future upgrades of wordpress

===

Secondly, I want to add autoclear

onFocus="this.value=''"

to the form, however when I do, the site breaks, the code is

function get_search_form($echo = true) {
	do_action( 'get_search_form' );

	$search_form_template = locate_template('searchform.php');
	if ( '' != $search_form_template ) {
		require($search_form_template);
		return;
	}

	$form = '<form role="search" method="get" id="searchform" action="' . home_url( '/' ) . '" >

	<input type="text" value="search the site ..." name="s" id="s" />
	<input type="image" src="/i/search-btn.png" alt="go" id="searchsubmit" > 

	</form>';

	if ( $echo )
		echo apply_filters('get_search_form', $form);
	else
		return apply_filters('get_search_form', $form);
}