Early spring is a good time (right now). Cut it back a little past where you want the ultimate size to be to allow for some regrowth. The plant will respond with vigorous new upright growth and you will need to cut that back a little to slow it down and force more branching. By the end of the season you may have trimmed the shrub back 3 or 4 more times but will have something more to your liking.
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