If you are serious about keeping a yeast bank, you should look into plating and slanting your cultures. That being said, if you can brew with a culture at least twice a year, I would just store a small crop in your refrigerator and use that crop to make a starter when you are ready. I have had crops last over a year. Granted, they needed to be nursed back to health, but brewing yeast is a lot hardier than we give it credit.
Here's a photo of my last yeast bank on slant:
Today, I am just looking for a lager yeast culture and an ale yeast culture that will cover 80 to 90% of my brewing (I brew mostly continental lager and British-style ale). That way, I can serially repitch crops if I brew at least once a month. Lager strains do not have the genetic variation of ale strains, so find one that you like and serially repitch it. A lot of breweries settle on BRY-96 or one of its derivatives such as "Chico," BRY-97, WLP001, and US-05. I want a little more character than these cultures provide. I just pitched WLP006, which is a culture with which I have no experience. It is a vault culture, so I will be forced to keep it alive if I like it. Hopefully, Verdant IPA works out when I finally get around to using it. I am brewing more during the warm months these days, so having dry cultures that work for me would a godsend. Diamond Lager looks like it is a keeper so far.