A belt can accentuate, and/or detract from a body shape so the key is to know what shape you are and what your proportions are so can work out the look you are trying to achieve.

Hourglass Rectangle Triangle (Inverted Triangle) Apple Pear

Pear shapes are usually the hips are wider than the waist and shoulders.  Use belts on the waist to give the impression of more volume in the upper body.

Apples are typically thicker through the hips, waist and bust. Belts are the perfect accessory for an apple shape.  Make sure you belt at the waist, not on the hips, the aim is to create an illusion of a waist.

Inverted triangles have broader shoulders than hips. The aim is to gain more on your hips so you can balance them with your shoulders. A belt will help with this. Especially good for those with a shorter stature.

Rectangles have the same width in the shoulders, waist and hips.  Adding a belt and doing it up tightly will break up the line between the shoulders and the hips. Cinching in the belt will give you the illusion of a narrower waist. Rectangular figures can wear a contrasting belt to give shape to the frame.

The hourglass shape is a well defined waist with hips and shoulders approximately the same width. Adding a belt on the hourglass waist will accentuate the small waist and adding a belt on the hips will also make the waist look narrower.