Allow me to clarify about the sand. Yes, you risk making concrete when you mix sand and clay. Not a good idea! When I do this, I replace almost all of the clay, so only maybe 10-15% of the soil that is left is clay. At that point, I feel that the clay is broken up and distributed enough to not make concrete. But I could be wrong! The aim of adding the sand is to improve drainage, so please do substitute something else for drainage, or just leave it out. The most important part of the mix is the organic matter. That's really what will improve your soil.
I found this article on NGA, it might be useful to you also.
http://garden.org/learn/articl...
