Light, I guess, is the biggest potency detractor.

http://www.erowid.org/plants/cannabi...adation1.shtml

Stability of cannabinoids in dried samples of cannabis dating from around 1896-1905 Harvey DJ
J Ethnopharmacol 1990 Feb;28(1):117-28

Abstract:
Cannabinoids from three samples of cannabis obtained from the Pitt-Rivers Museum, Oxford, and dating from the turn of the century were examined by gas chromatography and mass spectometry for the presence of cannabinoids. Although the samples were from different geographical locations, the profiles of constituent cannabinoids were similar. In common with other aged material, most of the cannabinoid content was present as cannabinol (CBN), the main chemical degradation product of the major psychoactive constituent, delta-9-tetrahydrocannabinol (delta-9-THC).