3. Fit is Everything: Jacket

Shoulders. It's simple: The suit's shoulders should fall in line with the top of your own, not halfway down your upper arm. Anything bigger makes you look like you're a linebacker or worse, wearing your dad's jacket. Properly fitted shoulders are the foundation of any tailored jacket silhouette.

Length: Put your arms down at your sides make a fist. At the absolute longest, the jacket should hit your knuckles. If the jacket goes past that, ask the salesman for the same size, but in a short.

Back and Chest: These areas should be snug, but not restricting. You should be able to fist pump with ease.

Stomach: Put your fist between your stomach and the jacket button. If it fits between and there's excess space, it's time to go down a size.

Sleeves: Let your wrist bone be your guide. The tux sleeves should hit at this point, and if you're trying it on with a dress shirt—which you should—about a half-inch of the shirt should fall below that point.