What branches should you prune? When should you prune?

Leaving growth on lower tree trunks for one to two years results in increased trunk size and strength.
It is preferable to prune trees while they are still fairly young (but older than one year). This will minimize the exposed or "wounded" area and speed healing.
Use appropriate tools that are properly sharpened. This will expedite the pruning process and will minimize damage to trees and shrubs. A pruning saw is best for limbs over one inch in diameter. Pruning loppers are good for mid-sized limbs, 1/2 to one inch in diameter, and pruning shears are best for thinner branches, 1/2 inch in diameter or smaller.
Be alert and careful. Wear protective gear: a hat, glasses and gloves. Don't work when you are tired.
Do not work near power lines. For large or difficult pruning jobs, consider hiring a certified arborist.
Prune deciduous trees during their dormant period.
Prune trees sparingly during the summer to avoid sunburn on trunks.
If a large pruning job is necessary, complete it in stages, preferably over a period of a few months. Don't remove more than 1/3 of the plant's mass during any one season.
Use of pruning paint or sealant is not recommended.
When in doubt, don't prune.