I used to have one and never really sussed it out so I resorted to pruning out a 1/3 every year in the spring. If you prune after flowering you lose the berries. I also tried after the birds have stripped the bush in Autumn but I got few flowers. so i resorted to the 3 yr cycle 1/3 every year.

Why do you prune it at all? Just to keep it small? Interested because I have just got one

Well, I have to prune it because I foolishly planted it by my back door, not realising quite how vigorous a plant it would turn out to be. If I don't prune it I won't be able to get out in my garden at all by May to see it's lovely flowers, however if I do (like this spring) then I don't get the lovely flowers which inspired me to plant it in the first place!! Catch 22! Maybe I'll try the 1/3 idea and just trim the side by the door? Thanks for your input!

i did mine just to keep it in the space where it was.

the 1/3 rule works well with many flowering shrubs which later have berries.