How do you put a softener out of service without damaging it?

* Simply "by pass" the water flow to the softener or shut water off.   
And unplug electric. If it the tank is in an area that could Freeze, move Tank to area that Can't Freeze.

Freezing can damage the Resins, Tank, and / or Valve.
* When you are ready to use the softener again, besides turning the water back on, restoring electric to control, and setting time of day,  you will want to manually start a regeneration of the system to "freshen" up the water in the Tank, before using the water in the home.