I prune my neighbours Forsythia for him,once it has finished flowering,Chris,and just to a pleasing shape ? I don't think they have special needs to be honest,and I know it's never been fed or watered .It's on a very dry steep banking,and never fails to look lovely each year. :o)

As Bloomer says prune it as soon it has flowered. You can prune it hard to about 6-12" if you want to. The flowers for next year form during the summer on the new wood. So the more you prune it the more new wood and the more flowers.

Don't worry, they are very tolerant. The branches that flowered this year are the best ones to cut off.
Incidentally you can use the cut off pieces for pea sticks and some of them may well root (I discovered to my surprise...)

Thanks to ALL of you for your very helpful words. It obviously is a simpler and more straightforward process than I had got inside my head!
You explained it so very well and I am ready and waiting now for the flowers to disappear!
Thank you