OpenPGP embodies an encrypt once scheme for messaging. That means that a message is created, encrypted and is only decrypted when the intended recipient wants to look at it. It can never be found unencrypted at any other time.
Secure encrypted message archiving is then as simple as keeping the archived messages around. You don't have to do anything special. Reading those archived messages is simply the same process used to read them in the first place. Such messages can be kept on a server under the control of a untrusted entity with no fear that the contents will be revealed.
This is an example of a simple and secure approach made possible by the minimalism of OpenPGP.