Simply keep them sterile and sealed, and in a reasonable temperature range. They will keep for a long time.
It would be best to keep them in a cool, dry, sterile place (like kerbe said). But whatever you do don't put them in your fridge, as it's really dirty in there. If you really wanted to, you could buy a brand new fridge and use it exclusively for spore prints and the like, but NO FOOD plz.
