If you buy your mushrooms prepackaged, you can leave them in their packaging – it’s designed to keep your mushrooms fresh.  If you’ve bought in bulk, place them in a brown paper bag.  Plastic like Ziploc bags, will trap humidity and cause the fungi to decompose faster.  Once you’ve bought your mushrooms, you can keep them in the fridge for up to 10 days, but they are best used as quickly as possible.