I usually trim mine a little after flowering and then again in late September.We have just been forced to prune due to gales and one of the trunks being split.They are quite tough and often get damaged in windy conditions.

If its Buddleia davidii, take the tops off now if its in a windy situation, leave if not, and then in late March, cut right back to within 2 inches of this year's growth on the woody stems.