Washing greens before storing them will cause leaves to rot easily, making for a slimy mess. Even stored properly, these greens may not last more than 3-4 days in the refrigerator, so have a plan for using them before buying!
If you know you won't be able to use your greens in time or notice the leaves turning yellow and starting to wilt, consider prolonging their life in the freezer.
To freeze hearty greens: 1) wash the leaves thouroughly and cut off any overly woody stems (you make the call on what you think is too thick/woody); 2) roughly chop the leaves and stems; 3) blanch the greens by plunging them into boiling water for 2 minutes; 4) pull them out of the boiling water and chill quickly in an ice water bath; 5) drain any excess moisture with a towel, and; 6) place into a clear, labelled (item name, date), airtight container or bag and freeze.
*Note: Contrary to common belief, freezing vegetables actually preserves the nutrient value of fresh produce. Getting ahead of a rotten or wilted item by freezing it preserves not only the taste, but also the vitamin and mineral conent.