The oft-derided fruitcake is the baking world's equivalent of a fine spirit, capable of aging and growing in complexity for years or even a lifetime. Part of that longevity and complex flavor comes from the quality of its fruit, which -- like a wine -- is loaded with tannins and other organic compounds that break down and re-combine over time. Appropriately, spirits such as brandy, rum and whiskey, age-worthy in themselves, play a role by acting as a preservative when soaked into the cake.
Allow the finished fruitcakes to cool completely on a wire rack before you handle them. Once they're ready, wrap each cake individually in multiple layers of cheesecloth.
Brush each cake liberally with a pastry brush dipped in the rum or whiskey of your choice, usually the same variety you used in the fruitcake recipe. Alternatively, fill a food-grade mister or spray bottle with the spirit and spritz the cake thoroughly. The cheesecloth should be very wet.
Wrap the cakes in wax paper or plastic film to seal out any air, then over-wrap this initial covering with foil. Refrigerate the fruitcakes, because at this stage they won't contain enough alcohol to act as a preservative.
Remove the cakes from your refrigerator and brush them with alcohol again every 2 to 3 days. They won't "ripen" enough to be fully flavored for at least 3 weeks, the minimum aging time, and 4 to 6 weeks is better.
Preserve the cake for long-term storage after at least 6 to 8 weeks of ripening by wrapping it in lightweight cotton muslin, a sturdier and more absorbent fabric. Half-fill a cake tin with powdered sugar, and nestle the cake in the fine powder. Cover it with more powdered sugar, so it's completely buried, then close the tin. Keep it in a cool, dark place such as a cellar, where it won't be subjected to rapid swings of temperature. Open it, and apply more rum or whiskey, at least every other year.