Do you want it to grow quickly? If you don't ignore it year in or year out.
But if you want it to fill an area, it's simple. Prune back the flowering heads when the flowers have fade, This will encourage new shoots from the side and base. I tend to leave mine until the berries are ripe, then prune, harvesting the berries is easier and they're yummy.

I went away to look up the name and found same as I would say, if you have Japonica. However I didn't know the berries would edible for us humans. I leave them for the birds but now will try one. There is another lower growing supermarket carparkia type mahonia which can be scalped any old time by the looks of it.