I'd say a general one is the old "wear your clothes; don't let your clothes wear you". This goes beyond looking comfortable; it also means that your clothes should help bring out your personality and not look affected or fake. This is, of course, something difficult to fix if you can't see the problem yourself so I'll move on to some more tangible rules of dress.
1. Always wear good shoes, you can get away (barely) with a lot of ugly, ill-fitting pieces of clothing as long as your shoes are beautiful and, above all, well-taken care of. There is no escaping the ugly shoe.
2. If you are in a city you really don't need gigantic clunky rubber soles on your shoes. This is hands down the most commonly occuring fashion faux pas here in Sweden. Young businessmen in expensive suits with hideous clunky shoes made in that shiny plastic-leather material. Don't ask me why, some might say something about the weather here but the problem remains in the summers.
3. If you are wearing a suit, your entire outfit sholud be geared towards this fact. Every time I see a man in a suit with an Adidas sports bag, bright red skiing gloves and a Nike windbreaker he just looks very uncomfortable and ashamed in his suit. He isn't wearing a suit, he's forced to wear one by his wife/job/mother and looks like a child whose mother decked him out in a sailors outfit. Wear a coat, leather gloves and a briefcase or smart leather or canvas bag. This way you chose your own outfit and you can wear it like a whole, looking relaxed and confident.
4. Keep the shirt lighter than than the suit or suffer the Borat, never striped shirt with striped suit, if you mix patterns use one big and one small. Alan Flusser has a good run-through of these.
5. The fit, of course, is paramount but the materials play a deciding role as well. This doesn't have anything (or very little) to do with price. It just comes down to knowledge and experience. FEEL the fabric of the clothes you are looking at. I sometimes get looks because I'm constantly groping the shirts, trousers and coats like some kind of deranged blind man but after a while you get a feel for the clothes. You'll soon recognize if a certain wool feels nice and durable or if it will age quickly. The sensation of a shirts cotton will tell you how it will fare after 20 washes. It's not that difficult, just something you get a hang of after a few tries. And as I said, you'd be surprised how seldom the price tells you what your fingers do.
6. Nobody wants to see your hairy leg sticking out when your pants ride up. Use long socks of good quality that completely cover all skin in risk of being revealed.
7. No sunglasses inside. Ever. If you don't need them for the comfort of your eyes, take them off. If you believe you can bypass this rule by wearing "cafÃ© glasses" or whatever they're called, the semi-tinted blue, red or green ones, then... suit yourself.