There are a number of ways, but I will list some of the easier/ cheaper ones to start with.

1. Switch to a coco husk or mulch substrate ( EcoEarth/reptibark/cypress mulch/etc.) aspen is better than some but the substrates I listed above will do a much better job.

2. Cover part of the cage's screen top with aluminum foil (no more than 70-80% though as your BP needs air to circulate.

3. Get a larger water dish

4. It may be a pain but the cheapest/simplest way to increase cage humidity is to regularly mist the cage with warm water.