One method for storing kefir grains for periods of up to 2 months, is by freezing spare grains.
To freeze effectively, wash the grains with pre-boiled COOLED water. Pat-dry the grains between pre-ironed cooled white toweling to remove excess moisture.
Place the grains in a jar or in a plastic bag, seal and put in the freezer. With milk grains, first add some dry milk powder, [DMP] adding enough to completely cover the grains with the DMP, seal jar or bag and then freeze.
DMP is mixed with the milk kefir-grains as a cryoprotectant agent to prevent freezer burn. Although I've found that kefir grains are viable for up to one year with this method, this length of time may completely remove the yeast component found in healthy kefir grains [if frozen for longer than 2 months, but not specifically]. Because of this potential, freezing kefir grains as explained above, is best performed for a period of no longer than 2 months.
If DMP is omitted for freezing milk kefir-grains, then a period of no longer than 1 month is recommended. Otherwise the yeast component of kefir grains may become damaged, especially if continual partial thawing and freezing due to poor freezer mechanism is involved.
To reactivate frozen kefir grains, thaw by placing the grains in a glass filled with cold water for a few minutes. Place the grains into a strainer and wash off any powdered milk that's adhered to the grains with cold water. http://users.sa.chariot.net.au/~dna/Makekefir.html#Storing_kefir-grains