Getting plants to flower for longer
Many plants can be encouraged to produce better and longer-lasting flower displays, which = deadheading. A plant produces flowers in order to reproduce. Once a plant has flowered and fertilisation has taken place, it will to put most of its energy into producing seeds at the expense of any further flowers. You can stop this by simply cutting out the flowers as they start to fade. The plant will then continue to bloom because it's waiting to get some fertilised flowers. Also, some flowering plants, like Columbine and Japanese Anemone are worth deadheading as they can self-seeding all over the garden.
Deadheading flower spikes
Many plants that have flowers on tall spikes, such as foxgloves and Lupins, can be encouraged to produce a second flush of smaller spikes later in the year if they are deadheaded. When the last of the flowers start to fade on the individual spikes, simply cut the whole thing off to just above the highest side shoot lower down on the stem. If you are lucky, side shoots will develop and these will flower towards the end of the summer.
Cut back early flowering perennials
Many of the early-flowering perennials such as Brunnera macrophylla, early flowering geraniums like Geranium phaeum 'Samobor' will have already flowered and may well be leaving a hole. Cutting them back hard to the base as soon as the flowers are over will provide a fresh crop of foliage and in some cases a second round of flowers later in the summer when things lose that fresh green that is so plentiful now. If it is dry, water thoroughly immediately afterwards, and in a fortnight new growth will cover bare ground.