Ask HN: What's the best way to store private key files?

So I have a few private key files. I REALLY don't want to lose them. How should I go about backing them up in a secure way? Just getting a thumb drive and a lockbox at the bank? Is there some service provide like Lastpass that does this really well? Should I just make a Lastpass note and dump the password protected key text in there?

I mean, I'm not sure what the security implications are of each given method and I can't find much in terms of a good answer by googling. There are some suggestions about formats to store the key files in and how to secure them on your workstation, but not a lot in terms of permanent, durable storage.