I kind of like the idea of moving the truck in front of the plane, and waving back and forth until both understand that the it's all done, then the pilots can watch the truck drive away.
If anyone is in a position to propose this, it might not be a bad idea.
There are a few airports, in Germany and elsewhere, where one of the trucks drives around to the front of the aircraft with a nice big, easy-to-read sign with the de-icing code on it (ie. Start/Stop times, Product, mix ratio). Always clearer than radio, and good confirmation the process is complete.
Best Practice?