Digital negative?
If you mean the hard copy print-out or screen display of an electronic picture file then archiving is as simple as photographing the hard-copy or screen with silver based light sensitive film and processing archivally.
If you mean deeply archiving the electronic picture file itself then you have to use the most stable CD, DVD, hard drive, whatever, storage medium you can get plus you have to store that thing under the most stable and benign conditions. Then you have to archive the software that turns the electronic file into instructions for a display device. Then you have to archive the computer, printer, or display device itself. Oh, and then you may as well archive electricity. Two hundred years ago no one used electricity and two hundred years from now something else, dark energy perhaps, may power the world.
In the end it could be that the digital domain is best suited for ephemera, entertainments, joyful diversions, and amusements unconstrained by material costs and the laws of matter and physics. By analogy, soap-bubbles can be fun to play with but who wants to archive them when they can be generated anew whenever needed?