Use a room humidifier to increase the air moisture in your grow room. You will need to periodically fill an air humidifier with water and have an available electrical outlet for this machine to work.
Place trays filled with pebbles and water underneath your plants. Check and fill the trays with water as needed. Make sure you only fill the trays up with room temperature water, just below the pebble line.
Mist the plants with a spray bottle during the morning hours in order to raise the humidity levels around the plants. Do not use this method if the plant species you are trying to grow are particularly sensitive to constant water exposure.
Group the plants together in the grow room to raise the humidity. Check the humidity levels with a hygrometer daily and increase or decrease the grow room humidity as necessary.