The best way to do it is manually. Spray your tank daily or bi-daily, and open up vents if you see that it is too high. Use any humidifier guage.
The alternative is to spend $80 on a humidifier control, which is a big ugly box that would have to sit inside your tank. Then you have to spend another $80 on a humidifier device of some sort and another $80 on a dehumidifier device of some sort...
The sell a full setup like you see at petco, for around $500. Any of those will cost you a small chunk of change every month just to run.
Keep some clump mosses and blanket mosses to regulate the humidity. They will release more water when there is less in the air, and less water when there is more in the air. (These are pet frogs, not frogs right from the amazon... They already have better conditions then they ever had.)