Freezing works well. I have Agfa Color paper, opened and later frozen ~ ten years ago. Still perfectly OK.

BTW - Hats off to Ole for his "Butane" tip. A Tetenal RA-4 5 liter kit opened in March of this year and religiously protected from oxidation by a 3 - 5 second injection of butane from my Bernz-o-Matic Soldering Torch was just used up three days ago. Still *fine* after all that time stored (partially filled bottles) in my darkroom at 70 - 75 degrees F.

No, NO - I DON'T light the thing before use ... Shee...!