The standard method of preservation of psilocybe mushrooms is to dry them using very mild heat to a crisp crunchy texture and to pack them into airtight containers (such as plastic bags) and store in the freezer. In the usual home icebox the mushrooms will retain their potency for 12 to 18 months.
When I grew my little creatures according to the instructions in the Magic Mushroom Grower's Guide, which describes this method of preservation, I thought that perhaps the reason for loss of potency was, not moisture, but oxygen. A plastic bag will be impervious to water, but allow oxygen and carbon dioxide to pass through readily. (One way to keep a house plant happy while you go away for awhile is to enclose it in a plastic bag. The moisture will condense in the bag and return to the soil, and the gases of the air will still get to the leaves of the plant.)
A readily available gas which will not pass through plastic is Freon (trademark Du Pont, etc.), which is used in refrigerators for its density and low toxicity. (Before Freon the main refrigerant was ammonia, which is quite poisonous.) Freon is available in most camera shops as Dust Off or Vari-Air in aerosol-like canisters. (It is used as a dust-free gas blast to clean photographic films.) A canister of Freon costs about $5.00 and is sufficient for packing tens of pounds of mushrooms.
The Freon in Vari-Air is dichlorodifluoromethane (that's a carbon atom with two chlorine atoms and two fluorine atoms instead of the four hydrogen atoms in methane). An alternative to Freon, which should work just as well, is 1,1,1,2-tetrafluoroethane, which is available in big, inexpensive cans from Costco, called Perfect Duster.
A plastic bag filled with Freon will block the diffusion of oxygen into the
bag, and more importantly, if the bag containing the mushrooms is flushed
with Freon, the oxygen in the mushrooms will also be removed. I packed a number
of ZipLoc freezer bags with single doses of mushrooms into an airtight container
which was also flushed with Freon and duly frozen. Three and a half years
later I enjoyed the fruits of my labors with no noticeable loss of potency!!!
Using gentle dry heat (not over 100 degrees F.) dry your crop to a crispy
texture. Go to a photography store and buy a canister of Vari-Air or other
Freon-based dust removal product (the kind of canister with a trigger-type
nozzle is easiest to use). Go to the supermarket and get some 5" x 7"
freezer storage poly bags (these are more substantial than the ZipLoc brand,
with a firmer airtight seal strip).
Weigh out the mushrooms into single or double dosage units (I have found that 5 grams is a suitable single dosage unit) and put them into the storage bags. Flatten out the bag as much as possible, and close the seal until only 1 inch is unsealed (most easily done by inserting the Vari-Air nozzle into the side of the bag and closing as far as you can).
Then inflate the bag to about 3/4 full (i.e. to about 3 inches thick) and shake the contents well. Press out the gas and repeat twice more. This flushes out all of the air so the bag is entirely filled with Freon. Remove the nozzle and roll the bag up from its lower end to press out as much of the gas as you can. At the last moment, press in the sealing strip completely.
You now have a bag with one unit of mushrooms squeezed down to a minimal volume.
Find a larger container - I prefer to use pint or quart plastic bottles with screw caps - and pack the plastic bags as follows. First, drill a small hole (slightly smaller than the nozzle of your Freon canister) in the bottom of the bottle. With the lid screwed loosely on, blast the bottle for a few seconds with Freon with the nozzle placed over the small hole.
Please note that the bottle, even a plastic one, prevents the diffusion of air into the bottle. You can in a pinch use larger plastic bags but this will not be as convenient nor work as well.
Pack the poly bags into the bottle, and when full, screw on the lid loosely and blast the bottle again for a few seconds. Then screw on the lid tightly and put some tape over the hole on the bottom. I use the brown plastic tape used for sealing cardboard boxes (available at stationery stores or moving companies). Use more of the tape to seal the lid.
The final step (although if the bottle is well sealed with tape this is not really necessary) is to put the bottle into larger freezer storage bags (gallon bags work well). Store the bags (or bottles) in the freezer. There is now a triple seal in effect, two using Freon and the outer bag protecting from the ice and moisture present in home iceboxes.
When you want to use a dose, remove it from the bottle and repeat the blast and sealing procedure on the bottle. Be sure to let your mushrooms 'air out' before performing your sacramental rituals.
Freon is an extremely inert gas and will not chemically react with your mushrooms. Do not use other aerosol products, apart from the the Perfect Duster mentioned above, since many other gases are used which may not be as safe as Freon. If you are concerned about our atmosphere then Perfect Duster is recommended (it's the chlorine, not the fluorine, which does the damage to the atmosphere). If you prefer 'naturalness' it is quite likely that carbon dioxide will do as well - 'dry ice' is a good source of carbon dioxide, as are certain fire extinguishers. Remember that the gas you use must be dry!
May the Logos treat you well in all your hyperdimensional journeys.