Document-oriented databases, with their flexible schemas, provide a nice solution. We can have older documents which vary a bit from the newer ones in the archive. The lack of homogeneity over time may mean that querying the archive is a little harder. However, keeping the data is potentially much easier.
I think this is pushing the schema migration issue from data to code, which might actually be a good idea.
Original title and link: MongoDB Use Case: Archiving (NoSQL databases © myNoSQL)