Google is about to take over everything anyway, so why not entrust it to them (!) via some online storage solution...
...then again, how long should forever be...? How long are you expecting the world to last...?
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Ok, quality dvd media, and a digital tape back-up is your only realistic option, apart from a hard-drive that you or they keep backed up, and can access should anything go wrong. More you can´t do...but technology should evolve to solve this in time, as it has done in the past.