Be sure that the bag that you seal the document in, is made from Mylar. A regular plastic bag will cause the document to become brittle and more yellowed, in time.
If you want to really preserve the documents, put a piece of acid-free mat inside the bag, as well.]
I've had good luck finding all of these materials at local framing shops that do archival work, but archival-grade mylar bags can be found at many vendors online.
I have used a couple of websites, successfully:
http://www.universityproducts.com/ca...d=1271&navTree[]=1271&navTree[]=1334
and.....
http://www.talas-nyc.com/
Talas has GREAT information on the care and storage of MANY of the items that we collect.
Good luck,
Bob.
(NOTE: I am not affiliated with either of these companies)....