Next to paper or parchment (mentioned below), there is a technique, that uses microform. Here in Germany a lot of long-term archival work is done with this material. The information is scanned and "printed" on microfiches. In one image, there can be stored a lot (i think up to 16) pages and a lot of images are on one roll of film. Then these rolls are transferred into special containers and put in an old mine in the German "Black Forest".

There are to date more than 28.000 kilometers of microfilm with round about 850.000.000 images. They estimate, that even in 500 years there will be no information loss on these images.

German Link: http://de.wikipedia.org/wiki/Barbarastollen