Customizing Content Query Web Part

There are alot of places where it explains how to change the look of this great webpart, and well, i thk this[http://www.microsoft.com/belux/msdn/nl/community/columns/stevenvandecraen/contentquerywebpart.mspx]
is very well resumed, with almost everything you need

My first tip

Using ?contents=1 after an aspx page, it will send you to the web part maintenance web page, there you can find all WP in your page and those that arent displaying in your webpage, and might be in conflit with something.

something like this:

http://site/Pages/Default.aspx?contents=1