Fertilize the hedge to encourage further growth. I fertilized my hedge with compost. Between the fertilizing and the trimming, my hedge should be ready for trimming again soon. This is precisely what I want: as I continue my trimmings, the boxwood shrubs will fill in the hedge better and better.
How often should you trim hedges? The answer will depend on the type of shrub that comprises your hedge, as well as on the degree to which you are seeking a "perfect" hedge. If perfection isn't your thing at all, but you want a row of shrubs on your property for the sake of privacy, consider, instead, a "living wall" of shrubs allowed to grow into their natural shapes.