Why Prune?  It is important to have clear objectives of what you want to accomplish before you start pruning.  Understanding how plants respond and the proper time to prune is essential (see When to Prune).  

It is important to know which tools to use to make the correct pruning cuts (see Pruning Tools).  Pruning is the oldest and easiest way to control the size, growth, beauty and health of your plants and to renew or increase production.

Prune diseased, dead or damaged wood to maintain healthy plants.  When cutting out diseased wood such as fungal cankers or fire blight, the cut must be made in good, healthy wood, below the point of infection, with a sterile cutting tool.  

When planting a tree or shrub, you should prune to remove broken, pest-infested or crossing branches.

Here's Our Garden Gang's Pruning Primer.

Typical Pruning Needs Of A Tree

Broken branches - Unsightly and possibly dangerous.
Diseased branches - Removal isolates disease-causing organisms.
Crowded branches - Removal increases light and raw materials to remaining branches.
Non-symmetrical - Removal improves appearance.

Cut Close

Cut an unwanted branch in such a way as to leave the shortest possible stub, and preferably flush with the remaining branch.

Two Main Pruning Concepts

Heading Back increases the density of the  plant and makes it more sturdy.  Thinning will make a plant grow taller and more open.

Pinching

Using the thumb and forefinger frequently pinch back soft growth throughout the growing season to avoid future pruning, to redirect growth and to increase the density of the plant. Pinching is also useful for disbudding flowers and thinning fruit.

Pruning Deciduous Shrubs

Remove all broken, diseased and crisscrossing branches. Remove a part of each long shoot that may spoil the shape of the shrub, and prune down to ground level about one-third of the oldest branches.

Choosing The Correct Bud

Prune near a lateral (side) bud that is pointing in the direction that you want the subsequent branch to grow. Cutting of a terminal (end) bud will cause the nearest lateral bud to inherit its strength and direction.

A Proper Cut

Support the branch below where the cut is to be made. Cut at a slant in the direction you want the new branch to grow.

The Cut In Relation To Buds

Too slanted - Exposes too much surface area to damage.
Too long - Can cause dieback of the stub.
Too short - Will interfere with bud growth.
Ideal - Cut from opposite the base of the bud slanting upward to the top.

Removing Heavy Limbs

Use a 3-cut technique to avoid damage to a tree by splitting. Cut at (1) under the limb, then at (2) above and further out to remove the limb, and at (3) to remove the stub. The heaviest limbs may be supported by a rope. Always use proper safety procedures.

The Ideal Hedge Shape

Prune hedges narrower at the top to allow sunlight to reach the bottom foliage.  This will keep the hedge thick and bushy.

Do's

• Always prune away dead, broken and diseased portions of a plant at any time.

• Cover cuts of 1 1/2" diameter or more with a protective wood compound.

• In general, prune weak plants hard and vigorous plants lightly.

• For safety and ease of pruning, use the correct tool for the job.

• Keep your tools sharp and clean. Clean cuts heal quicker.

• Make a cut only with a good reason and with an understanding of what your cut will produce.

• Always use proper safety equipment when pruning

Don'ts

• Don't leave ragged cuts or stubs.

• Don't use hedge shears for general pruning.

• Don't prune with sprung, dull or improper tools.

• Don't do all your pruning at the same time.

• Don't expect pruning to compensate for defects caused by overcrowding, poor soil conditions, improper climate, etc.

• Don't assume that every good gardener is a good pruner. Check out casual advice before you prune.

• Don't climb trees! The hazards far outweigh the benefits. Call a professional, or use a long-handled pole tree trimmer.