Business casual and formal outfits benefit a great deal from a stylish waistcoat. For a casual look, wear it with chinos, an unstructured blazer, a knit tie and wingtips, while making sure to contrast it with your shirt or trousers.
As a formal garment, the waistcoat often comes as part of a three piece suit. Make sure it fits snugly to avoid looking boxy. Add some understated elegance to your style by wearing a waistcoat.