Sin #1- Bad Fit. Most people don't realize it, but the way their clothes hang on their body is actually the most defining aspect of their appearance.
Sin #2- Not Dressing for the Occasion. An over or under-dressed person makes everyone around him or her feel a little awkward. Know what you're getting into at various social and business events, and know how to dress for every level of formality.
Sin #3 - Mismatching Patterns. Patterns that don't go well together jar the eye but wearing nothing but solid colours can be boring. Learn how to avoid both!
Sin #4 - Mismatching Color. Forget "honey, does this tie go with..." Know what's just not going to work, avoid it, and get on with your life.
Sin #5 - Dressing Your Body Inappropriately. Some "looks" work well on certain body types, but seem ridiculous on others. Don't be tempted into a bad style just because it happens to be trendy you've got to know your limits.
Sin #6 - Choosing Quantity over Quality. A wardrobe stuffed full of bad clothes is no substitute for even a lean closet of garments that make you look like a million bucks. Be strategic with your purchases, and know the quality of what you're buying.
Sin #7 - Getting the Details Wrong. A well-chosen outfit can be marred or improved by details as small as the cuff links or the pocket square. Know what details people are going to care about, and how to get them right.