Once you star thinking shapes, you are hooked. Basically there are two traditions, the Japanese and the Western. The former is an ideal way to give round shrubs good shape. It even extends to trees. Pruning branches is a good way of making sure that a specimen does not obstruct a view, but filters it to the rest of the garden. At its most effective it results in cloud pruning, which means you can reduce a Leyland cypress to five or six long bare stems, with balls of green on top. More traditionally a Portugal laurel can be converted into a giant mushroom, with a bare stem and a wide head of foliage.
But the two most popular plants for topiary are box and yew, both excellent for creating ball, boxes, pyraminds, spirals, and even animals. You can buy template shapes (open 3-D frames) from garden centers, training soft growth through and over. Also, try pruning by eye. Take a young box cutting and tweak it into shape, using stiff wire to which you tie it. A long stem becomes a neck, the middle the body, and so on. Or grow the box into a decent-sized shrub and then start shaping.
ABOVE: A smart, symmetrical grouping of four birds, with a variegated ivy dove in the middle. Box and yew shapes need two trimmings a year, in spring and fall, to keep the shape and maintain a dense covering of leaves. Avoid tight-angled, intricate shapes. With small-leaved ivy you can create slightly better defined figures.
RIGHT: Topiary is about having fun. Traditional shapes such as this spiral are hugely impressive, but you can easily invent your own.