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.
Smart People I Know 
For the sake of argument I would just like to say that, to me he looks a bit rumpled! lol!
From the wrinkled (linen or rayon?) shirt & shorts to the ill-fitting and obviously well-worn straw hat. And while the boat shoe is great choice for Summer, I still squirm when I see barefeet in a shoe! Especially on a man moreso than a woman for some reason. lol! Otherwise, he looks great! 😉
Well, I agree: he does look great! 🙂