I use the new Olfa rotary circle cutter. It is currently 50% off at Joann fabrics. Here's a link to see how it works: http://www.olfa.com/App_Content/Edit...er-Default.wmv
For irregular circles, I cut my fabric into squares, stack them, and cut corners off to resemble hexagons. Then I singe it into a circular shape. They look great.
Hope that helps.