i wont tell like “first look find technique”, this some kind trying your luck, i think this is the most common technique used by all hacker,beside using script this more reliable way to find because sometimes we need to edit or explore more. if you want to try simple way to get a xss vulnerable site just put a simple javascript like this :

<script>alert(’Backtrackbox.com’)</script>

or

<script>alert(document.cookie);</script>

just put it as a parameter, and as often the most vulnerable side from a sites is a search form , but of course sometimes it work and sometimes not, or maybe need more exploring. this is sample xss vunerable from http://www.lapdonline.org ,even los angeles police department have a xss holes. (i just get it at darkc0de forum). or this one at nasa.gov