I am a relatively novice mushroom grower who has been using PF tek for about half a year now. My mushroom yields were always paltry due to a lack of moisture and a lack of heat (ambient temperatures around 62F).
The technique that I created is to take a fairly tall (8") and narrow (~4") glass jar and fill it most of the way with plain tap water, and place this inside my terrarium. I then took an ultra bare-bones light fixture, which consists of a bulb screwed into a plain brass socket with a cord coming out of it, and put it in the jar of water, and turned it on, and replaced the terrarium lid. The bulb floats in the water, and the narrowness of the jar keeps the top end of the fixture (with the electrical contacts) from coming in to contact with the water. The glass alone is in the water, and the warmth of the bulb causes a constant plume of warm moisture to arise from the jar, and also keeps my terrarium at a nice even 85F. As the water level falls, so does the bulb, which you will find sitting in the bottom of the jar if you let it run dry. This setup evaporates about a pint a day.
If you don't have a light fixture such as this on hand, just buy a plain brass light bulb socket from a hardware store (the kind you would put in a table lamp). It should cost $1-2. Then cut the cord off of some old broken appliance (or else buy one for another couple bucks), and attach the cord to the fixture. I use a 40 watt light bulb; this is for a 20 gallon glass aquarium, so experiment to find which wattage gives you the proper temperature. If you have hard water, you may have to clean the jar every so often.
Note: I was afraid that when I added water, the cold water would cause the already hot bulb to shatter, but this hasn't happened yet. If you use a larger bulb, the chance of this increases, so exercise caution when adding water.
Ever since I started using this technique, my cakes have been fruiting much more plentifully and for many, many more weeks, without drying out at all.