How To Prune Herb Plants
There are many, many types of herb plants...too many to get into all the pruning/harvesting specifics for each type here. But below are a few general tips for pruning/harvesting some of the most common herbs. 

Pruning is essentially cutting, pinching or snipping off leaves and some parts of stems of your plants to harvest for use and to encourage fresh new growth. 

Whenever pruning herbs, lay a piece of plastic, paper, or cloth beside and beneath the plant to catch the clippings. You can often use them for potpourris, mulching garden paths, adding to the fireplace for fragrance or to the grill for flavor.

Here are some general tips on pruning your herbs:

When to Prune

It's best to start pruning your herb plants when they are young. Don't wait until they've grown large or spindly. By starting early and pruning often, your plants will develop a fuller and nicer shape. 

Harvest leaves and flowers of herbs in the morning when their flavor and aroma will be the strongest.

Avoid ever pruning off more than one-third of a plant. Especially evergreen herbs, such as rosemary and lavender, should never be cut back by more than one-third their size. Doing so will often kill the plant. That being said, soft perennials, such as germander and oregano, benefit by being cut back by one-half their size during the rainy season.

Cease pruning of perennial herbs (those that will come back next year) two months before the average first frost date in your area. Late fall pruning can encourage tender new growth that can be damaged by frost. This sudden shock can kill an entire plant. 

Perennial herbs, those that come back year after year, should be cut back in late winter.

Thyme should be cut back in the early fall to maintain bushiness.

Let lavenders grow freely throughout the summer and then cut them well back at harvest in early fall.

How To Prune

Always prune from the top of the herb plant rather than the bottom. The leaves on the bottom might look larger and tastier but, regarding culinary herbs, the most flavorful leaves will be newer growth at the top or around the outside of the plant. 

To prune leaves can pinch them off with your fingers or use scissors, snips or bypass hand pruners when your fingers won't do. Fingers usually work best when you're pruning a very delicate herb. It helps to have some fingernails to make sure you can pinch the leaf or stem off cleanly.

Herbs with tough stems, such as rosemary and lavender, can be pruned with snips or hand pruners. Avoid ripping leaves or stems off as this can lead to harmful plant disease.

Other Helpful Tips

Unless your intent is to harvest seeds, avoid allowing leafy, annual herb plants like basil and cilantro to flower. This robs vital energy and the plant can and often will die quickly after flowering. It's just the natural cycle of many types of annual herbs. Annual simply means that the plant lives for only one year or one growing season, and usually ends it's life from flowering or when cold temperatures come. So, if you want your basil and other types of annual herbs to live longer, make sure to snip off any flower buds or spikes that start to form. When pruning these types of plants, cut them right where the leaf meets the stem.  

Woody herbs, which are often perennial, like rosemary and thyme, should be trimmed occasionally to keep a dense form. This way they don't become too woody or straggly.  

Allow Coriander/Cilantro to flower if you intend on harvesting seeds. If not, remove flower stems as soon as they appear.

Herbs growing closely together should be pruned/sheared frequently to allow for good air circulation.

Allow the caterpillars to prune your fennel if you want them to turn into butterflies.