I have a piece of foam for each plane. It has the model number on it. The foam is stored on the prop of the plane. When I go to fly that plane I remove the foam, change the model on the transmitter, and then hang the foam on the end of the transmitter antenna. When I switch to a new plane, the first thing I do is remove the foam and put it back on the prop. then I get the foam off the prop of the next plane, etc...

Putting the foam on the prop provides a visible signal that prevents me from starting a plane before I've changed the transmitter to the right model. I also follow up a model switch by standing BEHIND the plane and confirming that each control surface moves in the appropriate direction.