Conceal: If you want to appear narrower in the shoulders, wear dark colors on top. Raglan sleeves on shirts also make your shoulders seem slimmer by drawing your eye inward. Avoid wearing anything boxy since this adds weight and does not draw the eye to a defined waist.
Reveal: Wide lapels or epaulets on your jackets will widen the look of your shoulders. Also, wearing anything with strong shoulders and a nipped waist will accentuate your shoulders.