1. Make sure you are well hydrated. Drink water early (to avoid getting up at night to undrink

2. You can hydrate your nasal passages with saline spray or a netti Pot. There's also something called Ayr Gel which is a saline nasal gel that won't carry the risk of lipids in your lungs.
3. Consider a room humidifier to bring the ambient humidity up a bit (but it has to be kept very clean!)
4. Make sure you aren't mouth breathing, regardless of what type of mask you wear. That just dries everything out.