An unopened package of Polymer clay won't expire if it is stored in a cool location. If the package is already open, consider putting it into a plastic bag and storing it in a dark, cool place. Clay will start to harden and cure around 100 degrees Fahrenheit. Keeping your clay out of sunlight and sealed from air will help it to last as long as possible.The best way to stop Polymer clay from expiring is to keep it in an airtight, dark, cool spot.
If your clay seems hard or unworkable, try taking it between your hands and rolling it into a 'snake' to give it a little warmth, which should make it softer. Give it a little bit of work and time, it sometimes takes a little bit of work to get your clay workable, but if your clay crumbles instead of rolling, your clay may be expired.