Lavandula angustifolia is the lavender most often grown in North America. It is hardy to Zone 5. Trim it twice a year. Cut lavender back hard in spring, to perhaps half its height. This pruning promotes bushiness and removes dead tips that are common after winter exposure. Lavender is not an herbaceous perennial; it is woody and evergreen. Most of the buds it sets each fall for the next year's bloom are on its youngest woody stems.
So avoid cutting stems all the way back as you would a peony, which has buds belowground. Leave some portion of each leafy stem rather than stripping it of all its active growth points at one time. Lavenders occasionally have surprised me after very hard winters during which they died right to the ground. I wrote them off as goners while clipping them back to leafless shrubs of old wood-yet vigorous shoots sprouted strongly from the lowest dormant buds at ground level. As soon as the plant finishes it's main June-July bloom, shear it-that is, give it a haircut to remove spent flowers. Shearing may promote a second flush of bloom.