Harvesting Herbs

This is my first season as a herb gardener and, as a novice, I am having great success with the growing part: I'm almost overrun, and it's only July. My main concern now is how to harvest leaves properly to promote continued growth and , more importantly, not kill the plants. Do I pick leaves from the top of the plant or lower down where the leaves are older ? Should I pick the stems or just the leaves ? Is there a general rule for harvesting leaves and seeds or is the method plant specific ?
P.S.: My growing area is an outdoor herb garden containing a good variety of herbs, but the ones I am unsure of how to harvest are: tarragon, marjoram, basil, dillweed and fenugreek seeds, oregano, thyme, lavender.

As a general rule, you harvest plants that keep growing taller, by repeatedly cutting the plant down to about 6 inches (15 centimeters). Plants that form rosettes of rather low leaves, such as parsley are harvested by cutting off the oldest leaves for use, but always leaving several young leaves to allow the plant to continue producing food. For most culinary plants the leaves are picked off and used, while the tough stems are discarded. If the plant is only used to flavour, say a soup, then the stems are used as well, but all of it is strained out before serving the dish.
To harvest fenugreek and dill seed harvest the seed heads just as they are starting to turn yellow, dry and then store for later use.
In lavender it is the flowers that are used, so the inflorescenses are cut off when most flowers are open, then are dried in the shade and after drying the flowers are stripped off the stems and stored in an airtight dark container in a cool spot.
For all the other plants on your list, cut off anything above the bottom fifteen centimeters to use fresh or dried. Basil and tarragon are best fresh or frozen since they lose flavour very easily and quickly.