Dressing well in the summer heat?

[sushijerk]

My summer wardrobe consistly mostly of t-shirts and cargo shorts. If I was attending a more formal function or going to a dressier place I'd be pretty screwed. What do you guys wear to look good/sophisticated during the summer?

I'm having a hard time finding slim black pants in a light fabric. I also want to get some of those white button down shirts that you can wear casually but also can throw a tie on. What are some good places to look?