Many growers say that their mushrooms, when kept in air tight glass containers, will last for 5+ years. Others recommend using carbon dioxide (CO2) for long term storage, to keep oxidation of psilocybin and psilocin to a minimum.

[Erowid Note (July 2016): First, nitrogen is a much better long term anti-oxidant storage gas than CO2. Second, there are widely available oxygen absorption packs sold for food that can easily be purchased online now. Third, CO2 is now widely available in small bottles from companies such as Sodastream and is more cheaply available in high pressure canisters (tanks) used ubiquitously to carbonate bevearges. Instead of using the dry ice method below, CO2 can be captured directly from a pressurized canister as long as the pressure is safely stepped down first.]

Since most people don't have a tank of CO2 available, dry ice can be used as the CO2 source. Dry ice is frozen carbon dioxide, so evaporating it at room temperature will produce CO2. It's generally quite inexpensive...somewhere on the order of $3 for 10 lbs.

Some people have mentioned a concern that commercially produced dry ice may leave a small amount of acetone residue when evaporated. Apparently acetone is sometimes used in the production of dry ice. It's possible to just ask the dry ice source about contaminents...by asking whether it's safe to put their product in a beverage to cool it.

Assuming the dry ice is contaminant-free...

1. A small pice of dry ice can be placed in the bottom of an empty mason jar. It takes about 3 cubic centimeters of dry ice (about the size of a standard die) to create enough CO2 to fill the mason jar.
2. Mushrooms can then be placed in the jar on top of the dry ice. As long as the CO2 is pure, it's ok for the mushrooms touch the dry ice. Alternately, the mushrooms can be put in a plastic bag inside the jar so they don't touch the dry ice.
3. The lid can then be set lightly on top of the jar, without sealing it, until the dry ice evaporates completely. Don't tighten the lid until it has all evaporated or it could explode. The carbon dioxide which is released during the evaporation process is heavier than air, so it will stay in the jar while displacing the air.
4. Once the dry ice is fully evaporated, the lid can be fully tightened.
5. The jar can be kept at room temperature until it's opened. The point of freezing mushrooms is to try to slow the oxydation process. In this case there is no oxygen present, so freezing would have little effect.