Attached Link: http://www.physicalc-software.com/tutorials/ao/

This is one of my favorites.  I don't pretent to understand all the ins and outs but Paul Kinnane's basic are a great place to start.  I fully agree with Karibousboutique too.  I try to go with AO surface nodes rather than blanket lights.  There my be surfaces that don't require AO, and using lights with AO increases Render times.

There is a bit more set up time placing all of the nodes but I think it's well worth it.

Best Wishes,
Rªnce