The most detrimental issue for the majority of men is poor fit. Their clothes sag off of them like hand-me-downs and create the perception that they are insignificant. While nothing is further from the truth, the problems with your clothes are almost always seen as problems with you--your clothes may be too big, but you'll look too small.
When you're out shopping for a new shirt or blazer, the first areas to check for good fit are the shoulders (yoke) and chest. On a man's garment, many things can be altered by a skilled tailor but the yoke and the chest shape the item and usually can't be changed. There is often an in-house ratio for the neck size to yoke and varies by brand. Be sure you can button the collar and still be able to slip two finger flush between your neck and the band.
For a shirt, good fit finds the seam attaching the sleeve to the body atop the outer point of your Acromion (see diagram). Likewise, a good check for blazers is to have a friend place their hands flat on the outside of your biceps and then move their hands up toward your shoulders. If, in this process, the shoulder padding of the blazer is lifted by their hands' motion, the coat is likely too big. In some modern ready-to-wear brands men are coaxed into buying clothes too small. If there is a noticeable dimple between the top of your bicep and the edge of the shoulder padding, the coat is probably too small.
Another feature of good fit is a raised or higher arm hole. This refers to the size of the opening at the attachment of the sleeve to the body and shapes the size of the rest of the arm as well as the mobility of the jacket. Possibly contrary to intuition, a higher arm hole is more comfortable for most guys because it mirrors the body's natural shape and allows for a freedom of movement that does not require the rest of the coat to bend and contort whenever you move your arms.
Contour is an increasingly popular element in men's formalwear as Italian style shapes American fashion. A great coat helps you look fit...even when it's not exactly true. The contour at the natural waist line is part of the visual effect that helps you appear tall and strong. I also prefer a double vent on both my blazers and suits--this type of opening creates good contour from the front and side profiles that is maintained while walking and sitting whether the coat is worn open or buttoned.
Waist and rise of the pants should coordinate with the cut of the coat. If the blazer is slim, choosing baggy pants will make your torso look awkwardly small and vice-versa. The coat is, by far, the harder garment to fit. Once you find someone (preferably us) that can help you get into a good jacket, the pants really are a simple addition.
While there are many, many more details that factor into a good suit, these are some of the basics that will help you begin to see fit in a whole new light and hopefully prompt you to pay a little more attention in selecting your next suit.
Joshua Davis is a full time image and lifestyle coach helping clients take control of their image as a tool for good communication.