At the end of class, if Mr.Potato Head has all of his parts, give the class a compliment when their teacher arrives to pick them up, or another agreed upon incentive. For smaller groups, you could pass around Mr.Potato Head at the end.
(I believe the original Mr.Potato Head classroom management strategy came from here, but please correct me if I'm wrong!)
How would you use Mr.Potato Head?
(Inspiration for Mr.Potato Head from here)