It is important the clay is kept air-tight. Pack the clay into cling foil and store the packed clay again in an air-tight plastic box. This prevents the drying of the clay. This clay dries very fast when exposed to air, if you work with the clay always use the amount of clay you need and store the rest air-tight. Even while working with clay, for example working with coloured clay, it is important that the same procedure is followed. Make a habit of this!