Proper pruning at the time of planting and as maintenance throughout the years is very important. It should be done to enhance the health and natural form of the plant rather than to try and alter it. At planting time, work to help your plant develop strong branching by pruning away any limbs that appear to be damaged or are crossing or rubbing the others.
Never make a cut that is flush with the surface or leave a stub when pruning your plant. Cuts should be made so the branch collar is left intact but no stub is left behind. If improperly pruned, the cut will invite disease and insects to the plant and prevent its natural defense system from functioning properly.
Maintenance pruning is needed to help maintain the health of aging plants. Generally it should be done in late winter or very early spring when the plant is dormant. However, there are exceptions. Plants, like lilacs that flower early in the year on old growth, for example, should be pruned immediately after flowering. Pruning later in the season will remove what would have produced flowers the following year.
Other plants may benefit from specific types of pruning such as heading back or thinning out to aid renewal. While some types of evergreens may be pruned during their flush of growth in spring.
If it requires continuous pruning to maintain the size or form that you desire, it is probably not the right plant for your purposes.
Some plants are very susceptible to insects and disease if pruning is done at certain times of the year. Oak trees should never be pruned from April through July because of the high risk of the spread of Oak Wilt disease at this time. If you absolutely must prune oaks during this period, use a non-toxic pruning paint to seal the wound immediately.
For general pruning of other plants (or for oaks at other times of the year), pruning sealers are not recommended. Wounds will heal most effectively if allowed to heal naturally.