It sounds easy, but many guys mess this up. Wearing your correct size will dramatically enhance your appearance and won't cost a nickel extra.
Make sure the shoulder seams of your new shirts and jackets fall at the end of your shoulders, not down at your triceps. Collars on woven shirts should close neatly around your neck. Polo shirts should end just below your waistline, not at your crotch.
Attention to the finer points of tailoring can help accentuate the body's natural proportions. A low waistline on pants will appear to elongate the torso, whereas a high waistline will shorten it. A spread collar will make your neck look more substantial, while a pointy collar will make it look longer. A jacket that drapes low appears to shorten your legs, while an abbreviated cut will lengthen them.