Do not watch only a change in your homepage but also
Some people say to me that they have their site totally under control. And when I ask them how, they say that they watch any change to their homepage every minute. Well, that is a start, but it only takes into account part of the defacements.
An enormous part of the defacements is by ADDING pages to your website. That is also the reason why many websites don't see this. A google alert isn't enough because Google only comes around every so many weeks and you shouldn't wait that long. You should really activate the logs that tell you when somebody logs in and when somebody adds a page or changes something on any other page. Off course the homepage is the flag for the defacer, but it looks like many off them are already very proud to have added a page to your website and don't bother to change the homepage itself.