Pruning shrubs consists in suppressing some parts with the goal of improving growth and flowering.
Although the formation of the shape has usually been started by the nurseryman, it is good to continue this during at least the first 2 or 3 first years following the plantation, in function of space, growth speed and the shape required.
Pruning for form
The first year after the plantation, between November and March, cut the branches by about half to encourage growth.
The second year, between February and March re-equilibrate the shape of the shrub if necessary. Cut the very vigorous branches shorter than the weakest ones. Reduce all the branches by at least one third, removing the weak twigs at the base of the shrub.
The third year, if the shrub is uneven or is still to weak renew the operation.
The following years, preserve the natural shape of the shrub. Remove dead branches, eliminate crossed branches (those that cross or are found to be growing side by side in same direction).
For good flowering, the flowering shrubs need to be pruned annually, at least to remove the dead flowers. If spring flowering await the end of flowering to prune, if not you will eliminate future flowerings. If summer flowering prune at the end of winter and always before start of fresh growth.
Attention Some shrubs do not tolerate pruning very well. Among others these are azaleas, camellias, Mexican orange (Choisya Ternata), Cistaceae and rhododendron...