Sunglasses must be worn with both caution and confidence. Not all sunglasses look good, and even the ones which do look good don't always look good with a given outfit. However, aptly chosen sunglasses make a powerful accessory. If you are a guy wearing a suit and you don't have on sunglasses, you are making a terrible error.* Also vests go well with suits.
* there are a few exceptions during which a suit may be worn without sunglasses, for example in a situation where you are playing piano to back up a comedy group and you don't want to totally upstage the comedians (that is an example which actually happened).