I've started using SSH keys instead of passwords just recently (thanks to GitHub, of course), so please keep in mind that I'm pretty new to this whole concept. Currently my keys simply lie under ~/.ssh, but I'm not sure if this is a good practice. E.g. if I have multiple machines, I'd need to duplicate my private keys, which I think is undesirable. Or, if my HDD goes kaput, then I'll lose those keys, which (I guess) is undesirable as well.
So, what are best practices on storing SSH keys securely, conveniently, and reliably?
Seems like using a smartcard is an option (see Smartcards for storing gpg/ssh keys (Linux) – what do I need?), is this the best one?
Update: The reason for the question was that many services (like GitHub, AWS EC2) provide guides on how to set up SSH keys for using the service, but little to no background (like, what to do if you already have a key generated by
ssh-keygen , what are recommended security measures). And it's unclear whether that info is in fact unimportant, or you're expected to know it ‘by default’.
To sum up answers up to this point (but please read them, and if you have something to add—please do): seems like in this case it's fine if you just leave your private keys in ~/.ssh, as long as you keep them from other people; but make sure that you have another way to access the service to upload or generate a new key if you lose one (which is normally the case).
 GitHub used to provide help on how to manage multiple keys.