Between mistings, humidity generally and inevitably will decrease.  It can be somewhat regulated with a moisture holding substrate such as eco earth, organic potting soil, or cypress mulch.  Humidity can also be better retained by minimizing ventilation, for example, covering most top screening with tinfoil.

    A water dish should provided at all times as well.  This helps to raise humidity and they can dehydrate quickly without it, even with mistings and high humidity.  It has been noted they frequent their water dish most often in the morning.  It should be large enough for the dragon to soak in if he so chooses.  A cat litter box will work fine for adults.  Cleaning should be done on a daily basis to keep the water fresh since substrate inevitably will get in, and a soaking dragon will often defecate in the water as well.

Hydration:
    Some frilled dragons will drink out of a water dish.  Those who won't will need other means of hydration.  They will often lap up water collected on plants and furnishings from an automatic or hand mister.  Some will be encouraged to drink from their dish if the water movement is disturbed by an airstone or small water pump (available for ~$15).  Most will eagerly drink from a large syringe when offered by hand.  Whichever methods work for you and your dragon, make sure it is getting plenty of water since dehydration will affect appetite and activity levels, and can lead to many health problems, such as gout.  A dehydrated frilly will have sunken eyes, loose wrinkled skin, and may have retained old skin that should have been shed.