"Hi guys, my name is Mark and I speak to you from the year 1930. I just saw a great flick called "Star-Spangled Squadron" and I'd like to hear some tips on how to preserve it for future generations, so that people can watch it after, say, 80 years."
"Hi Mark, this is the future speaking. I'm sorry to disappoint you, but the movie is lost to time..."
Nothing in this world can withstand the test of time. The greatest people die, the biggest monuments fall. Old knowledge is forgotten as the collective mind assimilates new data.
Networks offer data redundancy, but are you sure they survive EMP effect of nuclear strike?
Preserve physical medium. Factory-pressed discs are better than home-recorded DVD-Rs.
Keep an old rig running. Assemble an old computer running old OS. Keep it offline and not updated. As if the time stopped for this unit.
Keep related material as backup. Even if you won't be able to run the game, you can still present examples of gameplay on printed screenshots or captured videos.
Store one piece of each in an underground vault. Just in case China/Russia/USA/Burkina Faso/whoever wants to play Fallout as
LARP 
It may seem I'm joking about it. But have you heard about
http://en.wikipedia.org/wiki/Time_capsule ?