For non museum use, the popular answer seems to be hairspray. But for anything archival, that would be a no-no because of the chemicals.
Artists commonly use sprays to preserve charcoal drawings. Art supplies stores may carry a fixative that is "acid-free" or "archival safe".
Here is an example, although there is no mention on this if it is archival safe or acid free.
http://www.dickblick.com/zz217/02/
Susan Kline, M.A.
Library Assistant
Nebraska Methodist College
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