The simplest ways are to:
1. Use a different substrate, Cypress Mulch and Eco-Earth will boost your humidity more than just about anything else
2. cover part of the screen top of your cage. By covering 60% - 80% of the screen top you will hold in more the the humidity already in the cage
3. Get a bigger water dish/more than one water dish. The more surface area of water you have in the cage the more humidity will be released.
4. Mist the cage with warm water. There is a difference between a wet cage and a humid cage. By misting with warm water you will get more humidity per spray.