1, Daily MISTING with plain water helps.
2, wet rags and saucers of water in front of a fan helps.
3, Walmart has a VAPORIZER THAT YOU CAN USE FOR A HUMIDIFIER for $13.24 plus sales tax.
You need a cool mist humidifier or, if it gets cold in there the warm mist kind. Wet rags and bowls of water are kinda half-assed. Bigger/more plants will give off a lot of water.