I have noticed this the past few times I have been at the airport or the train station. People dress to "be comfortable", in their "juicy" sweatpants, but they end up looking like the just rolled out of bed. A rule I like to follow is to dress like you want to be treated and the author of the article below seems to agree and says that dressing up a few notches will get you better attention from flight attendants and security personnel on your travels.
Read the article here via Valet, via Gilt's MANual
The MANual also has a good list of tips on How To Pack For A Weekend Getaway.
P.S. Even though it is called the MANual, ladies could find some good tips in the travel article as well.