What is the best way to keep miso?

I put a layer of plastic wrap directly on the surface of the miso, and then the lid.
Keeps in the fridge for maybe ever, but i use mine frequently- especially for miso tahini salad dressing- so i use it within months.

In the old days, miso was stored in wood barrel or bucket with a fitted lid that kept direct contact with the miso to prevent drying out and stored in a cool, dark place.

Remember that the Japanese have been making miso for five hundred years, well before refrigeration invented.

Today, miso is sold in resealable plastic containers to maintain uniformly high product quality. Theoretically, one could probably store one of these in a cool dark place at cellar temperatures (50-55 degrees Fahrenheit) but why would you? It is far more convenient to simply reach into your fridge.

Miso will keep for a one year in the refrigerator per Shizuo Tsuji, founder of a famous cooking school in Osaka that bears his name and author of "Japanese Cooking - A Simple Art" (1980).

Miso won't mold; itself it is the product of fungus Aspergillus oryzae. There are some other beneficial microorganisms in miso beyond Oryzae which refrigeration helps to preserve.

Miso doesn't really go bad, but there is a loss of flavor quality after a couple of years.

Best to write the date of purchase on the package somewhere, just as a reminder to you how old it is the next time you open it.