I suppose it's the same as storing tea - airtight, opaque container, moderate temperature, etc. I did an internet search and found this statement..."can be stored for several decades in a airtight/air-feee container located in a cool dry place out of any UV light. This without any loss of potency."
Another post wisely recommends storing your kratom in small batches so you wont keep reopening your stash and reintroducing air and moisture which could lead to mold.
These all sound pretty good. Good luck. Will be trying the same thing, minus the freezer in case the electricity goes out. Then its not a cool, dry place.