As soon as the flowers have finished, so very soon, take out any dead wood, cut a few stems from the base, and then trim back the top growth to a good shape, whether the stems have flowered or not. Cut to leaf joints, where new shoots will come from.

I often take a look later on (in a month or two...) and snip off any wayward shoots, too! My shrub does usually also produce a few flowers in the autumn...