Presupposing that only a limited amount of knowledge could be saved seems wrong. You could bury petabytes of data in digital form, then print out a few books' worth of hints for getting back to the technology level necessary to read it.

Let's examine the problem in more detail: Different disaster scenarios would require different pieces of information, so it would help if you knew exactly what kind of catastrophe. However, if you can preserve a very large compendium of knowledge, then you can create a catalogue of necessary information for almost every type of doomsday scenario (nuclear war, environmental catastrophe, etc.) so that you will be prepared for almost anything. If the amount of information you can save is more limited, then you should save the pieces of information that are the most likely to be useful in any given scenario in "catastrophe-space." Now we have to go about determining what these pieces of information are. We can start by looking at the most likely doomsday scenarios--Yoreth, since you started the thread, what do you think the most likely ones are?