The straight edge that is the hallmark of the formal hedge depends most on what you do before you cut. Accurate measurements and well-placed stakes are key when you are trimming hedges.
John Sosnowski, horticulturist at the Morton Arboretum in Lisle, Illinois, recommends shearing the hedge in an inverted keystone shape, narrower on the top and wider at the bottom. The 6-inch slope is both attractive and healthy, as it allows all branches to get maximum sunlight. In this video, John demonstrates the procedure of laying out the stakes and string for trimming the boxwoods at the arboretum.
For more on this topic, see John's article "Trimming a Hedge."
Trimming a HedgeFor straight sides and a flat top, use stakes and string as a guide