There are several mistakes that men make when dressing that, if rectified will cost little or no additional money and elevate one's image:
1) Never wear a crew neck t-shirt with an open collared dress shirt. Wear a v-neck always. If you are wearing a tie and remove it, you are not showing underwear (which is precisely what an undershirt is.)
2) If you don't plan on wearing a tie, never wear a classic white dress shirt with a suit. It is perceived incomplete, as if you started with a necktie and removed it.
3) Rarely is the white shirt correct with anything other than a suit, therefore wear a sports jacket with a colored shirt, subtle patterned ones, or heavily more-textured options. It creates a more casual ensemble.
4) The correct shoe for a suit is a monk strap or lace up. Wear brown or oxblood dress shoes with your blue or grey suit. Black shoes are for after 6. The loafer is a casual shoe and should be reserved for jeans, chinos, and odd trousers, making it a great option for a sports jacket or blazer.
5) Addendum to #4...if you would like to wear a loafer with a suit, here's how it's done. Wear your summer suits with an open collared shirt, linen pocket square and loafers without a sock; footies or no see socks and speak with a drawl.... It's a southern classic.
6) Never match a pocket square with your tie. Sports jackets get a wool or patterned linen square. When wearing a suit, if you're unsure what to wear, you can't go wrong with white linen. Understated and elegant, it's a great finishing touch.
7) Don't wear your grey solid suit coat as a blazer. It will look like you think it's a blazer, and everyone will know it's your grey suit coat...
8) Don't wear a tuxedo to a black tie optional affair. Rather, wear your well tailored, black suit.