Two words. Research and planning.
What type of data is being stored? For how long? How long is it required to be immediately available?
I suggest stepping outside the issue as it were and looking at it as a whole. By mixing storage media between fixed and removable, including optical, DVD, and DAT tape along with an expandable SAN solution, you can have a very managable long term storage system.
The trap is not just trying to store too much for too long but the overhead of maintaining the storage.