You can install an inexpensive bathroom exhaust and duct it outside, also install a wall vent(or two) that is light tight(as per Ralph L.). I would put one intake vent near the ceiling, and another near the floor, and a power exhaust in the ceiling. Make sure to place the 'in' at the opposite end of the room from the 'out'. A weather seal around the door will help to shut out light. When you are finished, sit in the darkened room for at least five minutes before checking for light leaks.

Rick