there was a study in either hightimes or CC about last 6 months ago, he tried several ways of storing it. either in a dry cool place in a paper bag or in the refrigerator were the winner, trying to freeze it collapsed the cells. ref. did too after a while but most stayed viable. i have never done it long term but a paper sack worked well for me. only a couple of months. moisture and water will ruin them too. he had close up micro shots to show the pollen in different stages and storage methods.