If it's real hot you can go short sleeved, but not just a T with nothing over it, and definately not a buttoned down with short sleeves. Just get a polo.
If it´s too hot for a longsleeve, don´t wear a thinner longsleeve, thin looks grubby and trashy.
plain t shirts are for skater kids.
buttoned down shortsleeves are for geeks.
anything sleeveless is for stupid meatheads and fat alcoholic failures.
I´d recommend a polo shirt. I like the shirts of OeTTinger Bier
It`s the cheapest beer in Germany and if you order more than 10 cases they deliver free!
Nobody in the USA knows what it is anyway. They´d totally pass for a Fred Perry, which is a great brand.
I do insist you don´t wear polo´s unless you really sweat in anything else.
And one more thing-- TUCK THEM IN for Christs sake.
Don´t listen to so-called cool casual people who don´t, they´re merely clueless. Wear nice pants with a good quality leather belt, but not with a buckle that´s much bigger than the belt, that´s hideous.