New Rules for Dressing Up Well

Know your prospect's uniform.
Before you meet with a prospect, you should know that company's dress code. "Business casual" has a lot of meanings. Call the front desk at the company and ask what the company's dress code is and what the men and women wear. Or ask your contact. The point is, part of your responsibility is to understand that company's culture, including its dress code. Ask for examples, especially of the senior most person who will be in your meeting.
Dress one step up.
If your prospect is in denim, you wear khaki. They wear sport coats without ties; you are in suits without ties. The point is that you always dress one step further up the clothing ladder than your prospect, but not two. One step says that you respect and value them. Two steps can send a loaded message.
It's not just what you wear--but how you wear it.
Polished shoes, pressed shirts and well-fitted pants always.  At this point, some of you are thinking, "Does he really have to say this to people?" while others are saying, "Why do I have to tuck in my shirt?" But when your clothes are pressed, buttoned down and well-fitted, you convey that you are a person who pays attention to the details and are professional 
Grooming trumps style.
Even if you're wearing a great suit, if you've got a terrible haircut, you'll give a bad impression. As crazy as it sounds, everything on the grooming punch lists - fingernails, facial hair, haircuts and oral hygiene--matter. 
Know your company's uniform.
One of my clients makes sure that when his sales reps are making their sales calls, they wear a very specific uniform. (His company's clients accept this because they see it as an extension of the brand; the company sells safety products.) It doesn't matter if the reps are presenting in a board room or on a manufacturing plant floor, they wear the sample simple uniform. Obviously, if you work at this company, you follow this dress code in order to fit in.
Remember, you can dress in a way where your attire is the only message people remember, or you can dress in a way that takes nothing away from the message of value your company brings to them.