Mount a humidity gauge to the inside of your terrarium. A humidity gauge is easily found at pet stores, and it helps you decide if you need to raise or lower the humidity.
Use a spray bottle filled with water to lightly mist the interior of the terrarium. Be careful doing this, because overenthusiastic use of a spray bottle may cause the reptile to sicken due to excess moisture in its environment.
Set a large bowl of water in the terrarium, choosing a wide shallow bowl over a narrow, deep bowl. An increased amount of water surface in the terrarium increases the level of humidity.
Use an awl to poke a small hole in the bottom of a plastic bucket. Fill the bucket with water, and set it on the terrarium's mesh top. Set a bowl underneath the drip that results. This increases the humidity of the tank, and gives your reptile a place to drink.
Place a layer of thin mesh over the terrarium's mesh top. This reduces air circulation and keeps the air moist.