Working with herbs is an art and the smallest detail in harvesting and preserving them makes all the difference between success and failure.
Always harvest herbs in the morning, right after the dew has dried up but before the heat makes the plants release their volatile oils. Harvest fresh young leaves free of blemishes from areas away from roads and traffic. If you grow herbs for their flowers, always pick the flowers before they fully open. Never harvest plants on rainy days.
Set up the material to dry immediately, unless you want to make infused oil from the fresh plants, in which case you should allow them to wilt for a few hours, to get a good part of the moisture out of them. The best way to dry herbs is to hang them in bunches upside down in a dry, warm location. The best way to dry flowers is to lay them flat on a window screen, or anything that will allow good air movement under and through, and leave them undisturbed until they are completely dry.
If you have the room, keep the dried leaves whole, they maintain their medicinal and aromatic properties better that way. Store in dark glass containers or paper bags and never forget to label them with their contents and the date of harvest, you'd be surprised how much the herbs start to look alike after they're dried.
You can always make preparations, like infused oils, or tinctures (infusions in glycerin, vinegar or alcohol), for long term storage, but keep in mind that oils have a shelf life of six months at most before they go rancid.
To do so, tightly pack dried up plant material in a jar, cover it with your medium of choice, place the lid on the jar, making sure to insulate it with a sheet of plastic wrap to prevent the metal from reacting with the contents and keep the jar in a sunny window for a month, stirring the contents every day and changing the green material every three or four days in order to obtain a better concentration of active compounds. At the end of the month, strain the medium through a cheese cloth or a coffee filter and store it in a dark glass container in a cool dry place. Again, don't forget to label the container.
Unless you have specialized equipment to extract the essential oils, the standard method to extract the active compounds from herbs is infusion: teas, tinctures and oils. Teas and tinctures are for internal use, and you should only avail yourself of them if you are a trained herbalist or in the care of one, and only after consulting your physician. Oils are for external use, and will form the base of cosmetic or healing creams and salves.
Some herbs cross over from the medicinal to the kitchen garden, lavender, mint and thyme immediately come to mind, and those are best consumed fresh or as food seasoning.
You decided to start an herb garden? Here are a few tips. Most herbs like full sun (except a few, like mint and lemon balm) and a sweet soil that keeps moist but drains properly. If your soil is acidic, improve it with lime.
If you decide to grow a perennial herb garden, it is easier to start it from seedlings rather than seed, for two reasons. First, some of the perennial herbs, like rosemary and tarragon need to be started from cuttings anyway, germination is not always reliable and very young sprouts are vulnerable to anything from a late frost to dry spells or unseasonably warm weather. Second, starting with fully developed plants gives more control over the overall design and the assessment of proper spacing requirements.
If you are keen on starting herbs from seed, the annuals and biennial herbs are a good choice and will germinate well. Also for the likes of parsley and dill you don't have to worry about thinning, since they don't mind growing in compact carpet style.
Herbs are usually grown for foliage and they will seriously cut back on leafing out in order to produce flowers (unlike the plants grown for flowers, which will do the exact opposite in order to annoy you), so if you want, say, your basil to grow fresh succulent leaves, pinch the flowers or it will grow tall and spindly.
Don't harvest the large leaves that grow around the base of the plant, they are essential to keeping it healthy, they are how the plant stores food and energy. Always harvest the fresh growth, it tastes much better anyway and harvesting new leaves will encourage the plant to make more.
Some herbs, like dill and mint, tend to be invasive, especially in boggy soils. If that is a concern, grow them in containers or restrict their spread by planting them inside buried chimney flutes.
Always grow basil, not only because it is a staple of the kitchen garden, but also because it is easy to start from seed and very sensitive to draught. It will wilt before all the other plants and signal that it's time to water, and in this way it acts like a home grown living water gauge. Also, tomatoes and peppers taste better if you grow basil next to them.
Herbs don't need fertilizer. Keep them properly trimmed at all time, otherwise they grow gangly and woody, crowd each other and develop all sorts of diseases, from mildew to black spot.