It may seem odd to discuss the clothes that you wear and your grooming in a book about body language, but those two things are a vital component of the messages that you send out. How you choose to present yourself will affect how others see you. It's a well known fact that attractive people are perceived more positively than those that seem unattractive - but there's far more to being attractive than the looks that your parents passed down to you. You can affect how you come across to people by the care that you take with your appearance.
Consider the situation. Jon has an important sales meeting with a client. He stays up late preparing his figures and charts, and makes sure that he has all his sales material in his briefcase. Bill, the owner of the company to which he is pitching his product, is known to be conservative. Jon's own company is more casual and laid back, so Jon doesn't think much about it when he dresses in casual slacks and a golf shirt to make his presentation. Bill, however, sees things differently. In his eyes, Jon's casual attire marks a casual attitude toward work. He immediately pegs Jon as being less professional, and his entire estimation of Jon's presentation is marred by that first impression.
Sloppy grooming and clothing that is inappropriate for a situation send the message that you don't care enough about yourself and others to make an effort to look good. We like to believe that others consider us worth the time it takes to comb their hair and put on a clean shirt.