How can i get my humidity to stay up?

hi i have a water dragon and i am finding very hard to keep the humidity up in the tank .i have mess on the top so i have coverd that up it will get up to 50 then with in 10/15 min it has droped to 20 or even lower then i have to spray again and i have to spray ever half an hour please can you help as i dont wont to lose munch that his name thanks shaz