Wash herbs right before using and pat dry with a paper towel. Keep herbs in the fridge by wrapping them in a loose paper towel and storing them in a ziplock bag filled with air. Try to use before 5 days or they will lose flavor.
Heat usually kills the flavor of fresh herbs so try to add it to the end of the dish when cooking or on top as a garnish. Remember that casserole dish? Add a pop of green on top to make your dish more appetizing.