Wordpress search form + Clear text Field

form search

<form method="get" id="searchform" action="http://www.weedowonka.it/" _lpchecked="1">
	<input type="text" value="inserisci il testo da cercare.." name="s" id="s" onfocus="clearText(this)" onblur="clearText(this)">
	<input type="submit" id="searchsubmit" value="cerca">
</form>

main.js

function clearText(field){
    if (field.defaultValue == field.value) field.value = '';
    else if (field.value == '') field.value = field.defaultValue;
}