How? By following the example set by this man at The Sartorialist.
He is wearing a simple outfit, but his shirt is a very casual dress shirt: white, loose, and rolled up; it makes him more dressed up than someone in a T shirt or a polo/golf shirt. Likewise his shorts are also casual, but they are dressier than cargo pants or swim trunks. Finally, he uses accessories really well and they also dress him up. Not to mention that he is wearing shoes, not sandals or swim shoes. It’s a great outfit that would look great reading in a park or on a patio in terribly hot weather. Smart.