Cutting back encourages the growth of new foliage and makes plants bushier. More foliage increases the plant's ability to photosynthesize, which is turn contributes to a more rubust plant and increases flower production. Start by pinching the terminal growth or simply shear the entire plant. This forces foliar rather than flower growth, which makes the plant compact and bushy. No matter where you live, plan on shearing and pinching at least until the middle of July.
Many plants respond to cutting back.
In James's shade garden, you can see the flower and stalks of coral bells shooting up from the base of the plant. Once he has had time to enjoy them, he cuts them back. Blue salvia, which can be over-wintered in a pot, also needs to be cut back. Just remove the stalks of faded flowers, which will encourage new growth and more flowers. Plants tend to be healthier if you don't let the flowers go to seed.