I have found that warm water with dish soap works fine. I soak the parts in the soapy water for an hour or so, then take them out, brush them thoroughly with a soft toothbrush, rinse them well, and let them air dry.
Others use alcohol, or brake fluid, or whitewall tire cleaner. I don't that's necessary. I used whitewall tire cleaner once, and was never able to get any kind of paint to cure on the resin, even after waiting weeks after washing the parts. Tried enamel, lacquer, acrylics, nothing would dry on the surface of that figure, even after weeks of waiting for the paint to cure. I finally had to throw the model away- I could find no way to fix the problem.