Every now and then we get an inquiry from somebody wanting to know how to preserve their family archives. The principles of keeping anything for a long time are the same, whether it's clothes, food, or family papers.
In brief, organize what you have and store it in an appropriate way. This means papers and photos should never be kept in a place like a garage or basement where the air is still and damp--mold loves to grow under these conditions. Extremely hot and dry places like attics aren't good either because the organic matter that holds paper together will dry out over time and the paper will crumble. You also should protect your belongings from dust, light, and simple wear and tear by putting them in some kind of enclosure.
Treat your valuable family documents the way you'd treat your own body: cover them up and keep them in a dry comfortable environment. See these web sites for specifics on how to implement the strategies outlined above: