Boveda 2-Way Humidity Pack. 75% Humidity
Boveda's salt-solution 2-way humidifier pack. The 75% pack is rated to keep up to 25 cigars at 71-75% humidity for 2 months in a wooden humidor that is opened frequently - for a plastic humidor, make that 50 cigars.
Boveda has a patent on 2-way humidity control, and their packs can be recharged in any environment with a high relative humidity. Place a dry Boveda pack in an airtight plastic container with a small dish of water and put it in a warm place to sit for anywhere from a few days to two weeks. Refill the dish when it dries out, and your humidipack will be ready when it is back to its original "liquid" state. Boveda packs don't last forever, but can be refreshed for years before they have to be replaced.