Well...
A transom saver is supposed to transfer the weight of the motor from the transom to the trailer. The theory is that when you go over bumps and the motor moves a bit, the transom could crack. So, the device attaches to the trailer and grabs hold of the lower unit. Only problem I see is if the boat is not strapped down tightly, any movement of the boat (on the trailer) would transfer to the motor and to the trailer by the transom saver.
The other device Ospery and Friday talk about goes between the motor and the hydraulic tilt plunger (the left or right). This device keeps the motor weight off the hydraulic plunger so when the motor bumps around, the device takes up the shock (I believe is gives a bit, like a very hard rubber stopper). However, this device does not save the transom because it does not transfer the weight to the trailer.
Personally, I tilt up the motor, remove the tilt pin from the bottom hole and move to the top hole, and tilt back down until the motor just touches the pin. The skeg is higher than the trailer and I've never had a problem with it hitting the ground.