At the end of July or beginning of august, cut the long growth back to about 6 buds. Obviously leave any that you want to train or carry on growing.
Then, in Febuary, cut those stems back further from the 6 buds to 2 buds. This eventually forms a "spur" from which the plant will flower from.
A good organic mulch in spring and liquid feed every 2 weeks throughout the growing season will help too. Also, even though i grow mine on thick chalk, they do tend to favour acid soil.
Good luck.....mine was absolutely magnificent this year.