Dill leaves taste better picked just before flowers form on the plant. Start picking the fresh leaves just as soon as they are large enough to use. Pick early in the morning or in the late evening, clipping them close to the stem. If you prefer to harvest dill seed, allow the flowers to form, bloom, then go to seed. Cut the seedheads when the majority of seeds have formed--about 2 to 3 weeks after the blossoming starts--even though some tiny florets may still be blooming. Hang the seedheads upside down by their stems in a paper bag. The seeds will fall into the bag when they mature and dry out.
Freshly picked dill leaves have the best flavor. However, they keep for several days in the refrigerator, their stems in a jar of water and covered with a plastic bag. They store for several months if you layer them with pickling salt in a covered jar in the refrigerator. When you are ready to use the leaves, simply wash them and use them as fresh.
There are several ways to store dill longer term. Dry it by hanging bunches of stems upside down in a dark, dry, airy place until they are crumbly. Store them in a tightly sealed jar away from light and use within 4 to 6 months. Or use a food dehydrator according to instruction in its package. Freeze dill by cutting the leaves--long stems and all--into sections short enough to fit into plastic bags. Do not chop the leaves into bits because fragrance and flavor will be lost. They will keep in the freezer for 6 months.