I found a great new tool yesterday, the Helix shape template. I've been using cookie cutters to cut different shapes into the clay but they're never quite the right size for what I need so I end up having to accumulate loads of them.
The Helix shape templates have every size of circle, square, oval, etc. that you'll ever need. Simply place the template on top of your clay and cut around the shape using a clay pick or craft knife. You can get these templates from stationary shops such as WH Smith and Staples.