As a minimum, you need to prune off the flower heads as they die each spring to stop it wasting energy on seeds and encourage better flower buds next year. In addition you should prune out any dead or damaged stems right to the base.
Then, to keep it vigorous, prune out one quarter to a third of the stems every spring, going for the oldest each time. It should produce new shoots form the base. To keep the foliage colour, give it a feed for erciaceous plants every spring and then a mulch, after rain, of ericaceous compost to suppress weeds and retain moisture over the warmer months.
