Timing matters more than technique when it comes to pruning flowering shrubs.
Shrubs that bloom in spring, such as lilac, forsythia and azalea, set next year's buds over the summer. Prune them right after they finish flowering, or you'll cut off next spring's show.
Summer bloomers like panicle hydrangea, spirea and butterfly bush flower on new growth, so prune them in late winter or early spring.
Evergreen hedges can be shaped lightly in early summer and again in late summer. Avoid heavy pruning in fall: new growth will not harden off before winter and is easily damaged by frost.