Cut the greens off of the roots and store them in a resealable plastic bag. Leaving them out of the bag in the fridge will yield limp greens. If the greens are limp when you are ready to use them, you can refresh them by completely submerging them in room temperature water for an hour. This works really well if the greens are cold from the fridge. Use the greens within a few days of bringing them home from the grocery store. Fresh from the garden or CSA they will last about a week in plastic.