IMHO one of the best ways to do this is to provide IE6 users a complete but downgraded version of your site.
Most companies can't afford to just deny access to the swathes of people who use IE6, and of course users don't read messages, so just denying access to IE6 users won't work.
In addition to this many users don't have the ability to choose a different browser because of their network's group policy.
So I think the best you can do is give a (maybe deliberately) downgraded experience (not AJAX, simpler interface) and try to make it clear that this would be better if you were on a more modern browser. This is what GMail does, of course.
The important thing is to make sure the IE6 version still works and doesn't crash or render hideously. IE6 users will see that and think 'Oh, that website doesn't work, I won't visit there again.'
This should spur people to ask their IT department to upgrade.
It's not perfect, but then I doubt any method is.
Edit: Take a look at this blog post, basically says what I've said