In the context of online accounts and identities, what are some best practices to prepare for the one certainty we all face: not a one of us will get out alive.
The certainty of ultimate demise, be it biological or simply administrative (in a business context, employees retire or are fired, either of which being equivalent to dying from the employer's point of view), means that backups must be used, in particular for cryptographic keys which are used for encryption. For instance, if you receive encrypted emails, then those mails cannot be read without using the private key. Loss of access to the private key implies loss of access to the emails themselves, something which your successor may not like at all. In practice, if you use PGP for business reasons, then there should be a copy of your private key (the one for encryption, not the one for signatures) somewhere in a corporate safe.
As for your online estate (blogs, Facebook accountd, StackExchange reputation... are all some kinds of property to which you could attribute some value, and for which you have postmortem plans), the "normal" way of dealing with it is through a notary, who is a trained professional at such questions. Unfortunately, notaries are an institution from the Middle Ages, which is not structurally well prepared to cope with fast-changing data: you do not want to pay a visit to your notary each time you change a password. So I would suggest the following scheme:
You will probably modify the stored files regularly, which is a good thing, since storage mediums might not keep data unaltered for decades. GnuPG follows the OpenPGP standard, which is open and well supported, and thus there should still be implementations around in 40 years (or, at least, a working implementation would not be too hard to make).