Considering hand-written diaries, letters and so on can never be replaced, protecting paper collectibles from aging and the elements is crucial. If you collect old letters from the Civil War era, for example, the worst thing you can do is just leave them lying around for air and fingers to get a whole of.
Any paper product over time will begin to break down and deteriorate. The cellulose in paper essentially gets attacked by the oxygen in air. This causes the darkening or browning of old paper. In addition, light, too, can break day paper. Acid can get into the mix as well.
Paper collectibles such as books, diaries and other articles should be stored in dry and dark places. Light and moisture are not friends to paper. If individual letters are to be preserved, for example, it's a good idea to keep them in acid-free, or archival quality, sleeves or envelops. Limit touching as the oils and acids from the finger tips can cause problems down the road.
Other storage options for paper collectibles include: Polyethylene: Basically PVC sheets, these can keep some problems away from paper, but not all. They are ideal for keeping out dirt, oils and so on, but the PVC itself has elements that can lead to aging. If using this, make sure it's of archival quality and don't use it for the long haul.
Mylar: The stuff they make the vests out of is considered one of the best for storing papers. In fact, the U.S. Library of Congress swears by it. Mylar can be made transparent so documents held within its sheets can still be handled and read, as well.
Acid free items: Whether boxes or mounting boards or beyond, these are good for holding items, as well. Make sure they are of archival quality and do consider some of the better protectors for extremely rare or valuable pieces.
Not all antiques are made of glass. Some of the most interesting, in fact, are one-of-a-kind documents such as letters and diaries. To ensure these collectibles stay around for a long time to come, make sure they are properly protected for display or storage.