Pruning can serve many purposes. It can keep a plant’s size in check, encourage flowering or fruiting, remove or deter pest and disease problems or help to improve the overall appearance of a plant by changing its shape.
Good quality, well-maintained pruning tools are essential. You may need a pruning saw, long-handled loppers, hedge shears, pruners and gardening gloves. Be sure to keep all types of pruners sharp. Dull blades may crush a stem, leaving it vulnerable to infection from disease 
First observe the natural shape of a plant and then prune the plant in a manner that will allow the natural form to continue to develop. Remove any excess growth that obscures the basic pattern of its natural form. When pruning to shape, make your cuts above a bud or side branch that points in the direction you’d like the new growth to take. Try to eliminate branches that cross and touch one another. Crossing branches may rub together, suffering injury, and are usually unattractive, especially in deciduous plants out of leaf. Also, be sure to prune out dead, diseased and damaged branches whenever they occur.
Flowering shrubs bloom either from new growth or from old wood, depending on the plant species. Before you prune, it is essential to determine its type of growth. This way, you can avoid inadvertently cutting out stems that would give you a flower display. Most spring flowering shrubs bloom from wood formed during the previous year. Be sure to wait until these plants have finished flowering before you prune them. Growth that the shrubs produce after flowering will provide blooms for the next year. Most summer flowering shrubs bloom on growth from the spring of the same year. These are the shrubs you can prune during the winter
dormant season without sacrificing the next crop of blooms.
As a general rule, if a shrub flowers after mid-summer, it should be pruned in early spring. If it flowers earlier in the year, pruning should be done immediately after flowering.
Shrubs that can be pruned in early summer:
(after flowering)
Azalea Cotoneaster
Cytisus, Scotch Broom
Deutzia Forsythia
Kalmia, Mountain Laurel
Magnolia
Philadelphus, Mock Orange
Rhododendron
Syringa, Lilac
Viburnum
Weigela