Greg and Ron,
An added note, prompted by Greg's emphasis on avoiding sag: The National Cryptologic Museum at Fort Geo. G. Meade, MD, has a US Signal Corps "Battle Flag," professionally treated and mounted/framed. Reverse has a "lattice-work" of metal strips to apply equalized pressure to the flag, pressing it against the glass. Perhaps this is standard practice, but was my first encounter with the technique. (This flag, a rare "all five points" star denoting courage displayed under fire in the five named engagements appears to be of cotton or linen and was, as I understood, treated by TPA...about whom I have only heard good things.)

I might add that I was once told by a specialist at Smithsonian -- where silk CW flags are kept flat in "map case" drawers -- that there was no practical way to avoid silk deteriorating over time.