I use a sculpting medium named Fimo, a product of the german Faber-company. It has many advantages towards clay, oil-clay or other sculting-masses like magic sculpt or green stuff. It stays smooth for a very long time witout drying out, similar to oil clay, and you can sculpt even very small details. But in contrast to oil clay, it can be hardened. 30 min at 130°C and it gets a plastic-like consistence and is also very burstproof, so I can even sculpt very long and thin parts like tentacles (for which I had to use a wire-skeleton). Because I can harden different parts independently, I can sculpt complex parts like tentacles or hooks at first, without the danger of damaging them in the later sculpting process. Other mediums like magic sculpt have the disadvantage that they harden at the air, so you must be very fast, but Fimo keeps its smoothness for many weeks. And furthermore it is much cheaper, only about a third of "professional" model-sculpting masses.