Do not optical 'burnt' disks but regularly backed up hard drives. Use three - different makes - keep them in different locations (fire, flood, etc.).

If you must use optical media then use gold-plated disks, but see ...

http://uk.answers.yahoo.com/question/index?qid=20090425171413AAGucU0

Its a subject that is taxing the minds of even the technicians at the LOC. For better advice from them then join the mailing list at

ARSCLIST@LISTSERV.LOC.GOV

and ask there.

Also there is the digitisation list at:

history-digitisation@jiscmail.ac.uk