From 'wildchicken' site:
Can be left to get on with it, with just light pruning to maintain shape, or cut back hard in spring to restrict the size and keep the effect of the foliage. If pruning hard, carry it out in the same way as with Buddlejas, cutting back the stems back to about 20cm and then feeding - the plant will then grow very quickly and gain considerable height over the summer.

I just shape mine, so that I don't lose the lovely flowers! Pruning hard in spring will take the flower buds off, as well. If you only want it for the foliage, that's fine!

Try pruning out a third of the old growth each year. This way you will still get some flowers and also new growth.