How do I properly freeze and reheat a cooked, marinated steak?
I'd recommend letting it cool to near room-temperature, then place it in an airtight sealed plastic freezer bag before it dries out too much. You can spoon some of the juices from cooking into the bag with the steak so that when it defrosts again the juices will be in there working to keep it moist. 

When you defrost it, do so in your fridge leaving it overnight so that it defrosts gradually, keeping it in the freezer bag so it doesn't lose any more moisture. It should then be good to reheat, probably best in a shallow pan rather than a microwave to retain moisture.