Raising Humidity??

Whats a good way to raise the humidity in a small shed. Its 4x8x8 with a 1000w HPS that is air cooled, we also have 2 small desk fans blowing around the plants and a box fan kinda pointed up towards the light causing more heat to get away from the light and up towards the ceiling. Anyways, there has to be a easy cheap way to raise the humidity. It usually stays perfect, just sometimes it gets down to the mid-upper 20 percent. Thanks for the help.

I'm going to give one of these a try: Honeywell 2 Gallon Capacity, Cool Mist Humidifier - HCM-890 at The Home Depot