Storing them in milk under refrigeration works best for me. I dry off my goats in mid to late fall and they don't freshen again until April. I've had viable grains after storing then in the same milk for 5 months, but it is better to refresh the milk every month or so. I just don't like having to expose them to store bought P/H cows milk during that time because mine live off of raw goat milk the rest of the year.
I've also dried kefir grains in the past. That's a good thing to do to have an emergency stash in case something happens to your working grains. Like when your daughter throws the culture in the compost not knowing what it was.
. So far in reconstituting dried grains I found that they make good kefir, but they won't seem to grow and multiply. Not sure if that will happen in the near future.
I've never frozen them. I don't know if that would work or not.