Periodically I back up the keys within my servers and databases, and when I do, I blog a reminder here. This should be part of your standard backup rotation – the keys should be backed up often enough to have at hand and again when they change.
The first key you need to back up is the Service Master Key, which each Instance already has built-in. You do that with the BACKUP SERVICE MASTER KEY command, which you can read more about here.
The second set of keys are the Database Master Keys, stored per database, if you’ve created one. You can back those up with the BACKUP MASTER KEY command, which you can read more about here.
Finally, you can use the keys to create certificates and other keys – those should also be backed up. Read more about those here.
Anyway, the important part here is the backup. Make sure you keep those keys safe!