I'd recommend slicing only what you are going to eat and then wrapping the rest in plastic wrap or aluminum foil. You can also wrap the entire pan once cooled, before slicing, in plastic wrap or aluminum foil. These brownies will stay fresh for 3 days at room temperature, 1 week in the fridge, and 3 months in the freezer. To reheat them, just thaw them out when you're ready to eat and heat them in the microwave for 10-15 seconds.