Periodically remove the clay in a cool place, e.g. overnight in the fridge. But it is not always convenient because of the constant attention to the craft.
In the manufacture of large parts DIY clay can make them less plastic by blending it with materials that are insensitive to heat. For example, it can be small pieces of copper wire, and plastic insulation. Cut the wire with scissors and mix thoroughly with the clayom. Or for future product use wire as a frame.
Small pieces of crafts, such as grass, hair, leaves, stucco trees require different methods of hardening. For example, spray to the surface of the hair coat or transparent nail varnish.
You can resort to a method of modification by further refining the chemical composition of the clayand before you start sculpting. To do this in a saucer of water evenly spread out the clay thin pieces. Heat in a microwave oven to become fluid, resembling oil paint.
Remove the dish from the microwave. The resulting melt stir in water, then this mixture must be mash, pressing out the excess water. The weight put on the cold or in the freezer, and pour the water and enter a new one. So repeat several times, in fact, seeking thereby the gradual leaching of the oil plasticizers. The result of the clay will be much firmer.