Prune right after blooming, usually no later than June (at least in my
area).
Cut out dead wood and remove spent flower heads (not necessary for next
year's flowering but it tidies things up nicely). You can reduce growth by
no
more than a fourth to a third at this time, although I prefer to do major
renovational pruning in winter when the shrubs are dormant. You'll lose
some
flowering, obviously, but it is far easier on the shrub to do hard pruning
when they are dormant.

FWIW, I also do a pruning in early spring to remove dead wood. By this time
the shrub already has leaves and the flowers are already growing, so we're
talking some fairly obvious dead wood.

James