Mainly practice. I love working with it, but you need to learn exactly how it behaves. It starts out pretty much too sticky to use, then goes to a soft, squishy consistency that's perfect for making it conform to other surfaces. Now is also the time to blend it into other areas using a little water. Next stage, it stiffens up a bit, allowing you to push it into shape with your sculpting tools. You need to be careful removing areas at this stage as you can accidentally pull away more than you'd like. As it begins to harden, you can continue to work it with a scalpel and other metal tools, and it will still respond to water. I reckon you get about 25 minutes useful time before it hardens. Once it's hardened you can scrape, file and sand it.
Don't try and tackle large, detailed areas all at once, as you won't have time to work it. I also like to build up gross surface shapes, harden them and then apply texture and detail using a thin layer with lots of water. You can scrape off bits of the texture area because of the hard surface underneath.