Newplast is often used by the famous Aardman studio!
Newplast is a non-drying, non-setting modelling material. 
It is virtually identical to plasticine, and can be re-used many times. 
Newplast is sufficiently pliable to make a wide variety of models, but is firm enough to retain its shape indefinitely.
Be aware every new made batch can give a colour difference between the new and old stock!