Masking the smell with air fresheners isn't solving the underlying problem...you need something that will actually absorb or neutralize the odor. There are a few ways of doing this that have already been mentioned, but IMHO the simplest option is to use a charcoal based deodorizer.
Walmart carries Willert brand closet deodorizers in their laundry / storage & organization isle that work great (approx $5-$6). The office in our home is the catch-all for my collections...militaria, vintage baseball items, and vintage Hi-Fi stereo equipment...all of which tend to carry musty smells. After placing 1-2 of the Willert deodorizers in the room, there are no noticeable odors whatsoever. I typically keep one deodorizer on the wall shelving where my vintage baseball gloves are displayed to combat the musty old leather smell, and another in my walk-in closet where my militaria collection is stored and displayed on built in shelving (the deodorizers are small enough to easily hide from view).
Here's a link to the product on another site (price shown on Walmart's site is twice as expensive as in-store)... http://www.storables...deodorizer.html
Edited by coolhandluke, 25 February 2015 - 07:22 AM.