Thats a big question, especially in the scope of tape and tapeless projects. Here are some thoughts.
For your tape-based projects, as long as you’ve properly logged your tapes, all you really need is the FCP project file, and any non-tape elements (gfx, music, etc.). Generally this can all fit on an archival DVD or two. Simple.
A step above that is using media manager to create a consolidated project. Using the ‘copy’ and ‘remove unused media’ functions in the media manager, you can make a version of your project that discards the media that is not used in your final timeline. This allows you to have the project at your fingertips for quick changes, while greatly reducing the amount of archival storage needed.
Tapeless media has greater archival needs. Best practice is to make two redundant copies of the disk image of each card used. From there, you will extract the media into FCP and perform your edit.
As far as storage goes – external hard drives are very popular. Everyone has horror stories about this brand or that brand dying. They _all_ die. For critical archives, you should have a redundant pair of everything you want to save.
If you’ve got a lot of data to store, data tape formats like DLT and LTO are becoming popular for video people. They’re expensive, but if you’ve got the volume, it will save you money.