Umm okay, so remind me of the question again? I thought you said you got a slab of whatever this stuff was. Does mephedrone come in slabs?
Anyway, the general advice for preventing the degradation of chemicals is to store them with silica gel (the sachets you find in new shoes and beef jerky) and keep them cool, dry and out of direct sunglight.
We get them shipped from Amsterdam, come in foil wrapped since its illegial in the UK :/