Rather than have them be for fixed periods, have the registration fee increase quickly. Want a 5 year copyright? Sure $100 registration. Another 5 years? $1000. Another 5 years? $10,000.

This results in highly profitable media still being in copyright, but basically everything falls out somewhere close to the year 25 1m renewal mark, or soon after.

Also, you have to give a few master copies to the copyright authority, so that it's less likely for the media to become lost.