Hi Sarah. Calvin always tries to use fresh Plasticine. Try to keep it nice and clean too.
He bought some Wallace and Gromit modeling kits on eBay but the were no good as they were old, dry and crumbly when they arrived.
He takes his time and works really hard on them. He uses little cocktail sticks underneath the Plasticine to keep the arms, legs and heads in position. Hope that helps and good luck