Have two players facing each other, both on one knee. Have them throw back and forth without stepping or using the legs and make sure they follow through so their throwing hand is past their opposite knee almost touching the ground.
Have the players stand face to face holding their fielding arm at a 90 degree angle in front of their chest and feet shoulder length apart. Have them put their throwing elbow on top of the glove and throw a ball without moving their feet or the rest of their body, only their arm above the glove. In essence they are just flicking the wrist to throw the ball.
I've also done the towl drill and that is also a great drill to use.
Don't forget to run them to stretch out their legs and build leg strength.
