If a jacket has a breast pocket, there should always be something in it. Period. Ideally, it should compliment but not match anything on your outfit, but when in doubt, match your shirt color. Traditionally silk, a pocket square used to be placed effortlessly into a pocket and forgotten- looking sloppily like it just prevented a mess being sneezed in your hand. A modern man wears it neatly flat, exposing but a simple elegant line to compliment his silhouette. Buy cotton or linen as it tends to hold its shape better than silk. Check periodically to ensure it stays that way, but fix it only in the bathroom- never act like you care about your appearance for it spoils the effortlessness of your outfit.
P.S.- Round Toe. Not Square. End of Story
Suit, $296, by Kenneth Cole New York. Shirt, $45, by Nautica. Tie, $150, by Black Fleece by Brooks Brothers. Shoes, $680, by Salvatore Ferragamo. Socks by Punto. Pocket square by Paul Stuart.