My experience is that if dried carefully and packed in a specific way, the dehydrated shroom powder can retain its full efficacy for at least four years. The Anubis technique is to dehydrate the mushrooms over a very gentle heat over a period of about 24 hours - the mushrooms should never be more than slightly warm to the touch while this is happening.
When the mushrooms are absolutely bone-dry, reduce them to a powder in a Magimix or similar blender and then pack this powder very, very tightly into a glass container that has a very tight seal at the top (preferably one with a rubber ring between the lid and the top of the jar). DO NOT USE PLASTIC FOR STORAGE. My experience of even so-called hard plastic containers is that over a relatively short period of time they seem to slowly absorb air, resulting in the serious deterioriation of the powder within the space of a year.
When you finally open the glass container again months or years later, tiny little particles of shroom powder should spontaneously float upwards out of the container - this means that all is well. If the powder has gone sort of brownish and you don't have floating pieces rising up to greet you and tantalise your nostrils, then water absorption and resultant deterioration has definitely set in. It is very important also to force in as much powder into the glass container as possible in order to expel as much air as possible from the container before you seal it.
Using this technique, I have found the powder to be as potent four years later as the day it was packed. I personally think it is a mistake to store the mushrooms in their whole dried state as the mushroom in its original structural form is like a sponge just waiting to suck up whatever moisture is around. By powdering it, you remove its sponge potential.
I've never refrigerated or frozen my containers - just kept them at room temperature (ie around 25 deg C.)
* Reserection spelled intentionally to play with 'res - erection' for the phallic mushroom and to separate it from the very different Christain conception of resurrection.