All the wrong plants get shorn. Mow, blow and go gardeners are known for shearing everything that they can reach into creepy and unnaturally geometric shapes, usually flared out at the top, and bulging obtrusively into otherwise usable space. Yet, functional formally shorn hedges are passe and almost never shorn properly.
First of all, a formal hedge should be uniform, which means that all the plants that comprise it should be the same, and planted at the same distance from each other. Any plants within a formal hedge that die should be replaced with the same plant. A single English boxwood plant added to a Japanese boxwood hedge will always stand out with a different shade of green and growth pattern. Seedlings of other plants that grow up into a hedge should be removed before they become part of it.
Secondly, a formal hedge should be contained within defined boundaries. It should get neither too tall, nor too deep (from front to back). A fat hedge wastes space. There is no excuse for a hedge to encroach obtrusively over walkways, driveways or patios.
The vertical faces of a hedge should be completely vertical or slope very slightly inward at the top, but should certainly not slope outward to shade lower growth. A slight inward slope is more important on a northern face of plants that are sensitive to shade.
Hand pruning hedges (with hand shears) produces the best finish, but takes quite a bit of time for big hedges. Shearing with typical hedge shears cuts leaves as well as stems, so can cause foliage with large leaves to look a bit tattered for a while. Hedges that grow fast may need to be shorn twice or more annually. Some hedges can get shorn back aggressively once annually, and then allowed to fluff out evenly through the rest of the year. Bottlebrush and escallonia hedges can bloom nicely and evenly if shorn only once annually.
Glossy and wax leaf privet are among the most traditional of formal hedges. Glossy privet can get quite tall, but wax leaf privet actually has glossier foliage. Photinia and shiny xylosma have a bit more color, but want a bit more space (from front to back). Boxwoods make very nice small hedges. Pittosporums are available in various shapes and sizes. Pittosporum eugenioides gets quite tall, and can be kept quite lean. Pittosporum tobira is shorter and wider.