Pour a small amount of baby powder into a bowl.
Load a fluffy makeup powder brush with a generous amount of baby powder.
Brush the powder against the clay using small, circular motions. You can be aggressive: the brush is too soft to damage the clay. This works well for water-based clay, and is adequate for oil-based clay.
Purchase a can of compressed air from a computer shop. This product is typically used to clean electronics.
Hold the can upside-down.
Squeeze the trigger, allowing the liquid from the can to spray over the clay. This will rapidly cool and harden it for a short time.
Reapply this liquid as needed. This technique works well on oil-based clays, but can damage the surface of water-based clays by creating ice crystals in the clay.