preserving printed circuit boards
Usually you vacuum pack them with some desiccants in side.  Or in other words put the boards in a bag, throw in some moisture absorber packets (same stuff you see in vitamins that says do not eat). Then suck all the air out of it then seal it off.  I suppose if you were fancy maybe nitrogen would work too.

Also different finishes have longer shelf life, hasl or tin is not so great, silver immersion is better maybe you can get 6 months out of that, and then gold enig will probably give you a year or more.   But sealing them up will be the best.  Your manufacturer should be able to do it for you if you don't have a vacuum sealer.