I wish I knew the final answer to this one.
I keep poring through different books and they all have a different view. The book with the most scientific approach seems to be The Unofficial Guide to Walt Disney World. Their website is here:
http://www.touringplans.com/tp2/UG2_index.php?PageID=0
It has crowd predictions on a day-by-day basis. Fantastic you might think, except some of the web updates are completely different from those in the book. You wouldn't have thought they would change that much!
A note of caution though - The Unofficial Guide say that one of the best days to do MK is a Saturday and they claim evidence to prove it. However, of all the times I've been to MK, the worst day, by far, was a Saturday.

Seems to be okay for other predicitions though.
The park that has worked for me is Epcot. I don't think it is as popular at the weekend and its a big park, so it absorbs lots of people. Still ride your favourite attractions ASAP though