Cool and dry. I store my seeds in zipper bags in the lower shelf of the fridge, each with a little envelope of powdered milk in the bag to keep them dry. Silica gel is even more effective than the milk, but more expensive.

I use a paper evelope rather than plastic to ensure that they stay dry.

That works if you have a frost-free fridge, unlike my old thing! True, I am undoubtedly much in the minority.