I use the Repti-Fogger in one of my dart vivs, but it's mostly for asthetics. Like Paul, I put water features and have running water in all my vivs, even my green tree frog viv. The advantage of the Repti-Fogger over others is that it is external to the viv and you can have the fog enter the viv where you want using hoses. I have it coming in at the top in the back and it cascades down - pretty cool. The fogger designed to place in a dish within the viv can be dangerous to the frogs if they fall onto the ultrasonic plate.

My dart frog vivs are sealed pretty tight with controlled ventilation. That, combined with running water, keeps the humidity up pretty high even though we have an AC unit with a dehumidifier.

Putting water in the drainage layer can help if it is heavily vegetated because the plants will transpire a lot of water. However, without the plants, the water can't really get to the surface and evaporate unless you have a water feature. I use waterfalls, streams, or drip walls with an inexpensive powerhead and a short piece of tubing. Moving water increases evaporation and keeps the water from getting stagnant. It also oxygenates the water so you don't develop a nasty anaerobic layer, which can form methane and hydrogen sulfide (rotten egg smell), neither of which is good for animals.

To make my waterfalls, I create a sump in the drainage layer to place the powerhead on the bottom of the viv and cover it with a small flower pot. I run the tubing under the substrate to where I want the waterfall. I use cork bark for the water fall structure, drilling a hole the size of the tubing. Then you can either glue the cork bark in place against the glass or simply lean it up in a corner. You can dig a small pool at the base of the cork bark down to the drainage layer fill it with some nice looking gravel. I typically fill in behind the cork bark with sphagnum moss. Cork bark has natural channels running lengthwise, so it makes a perfect waterfall structure. If you have sufficient lighting, moss will flourish on a waterfall. All the materials for this will cost less than the Repti-Fogger, but either would work.