Clubbing / Party : For pants, you can't go wrong with dark jeans or dark khaki pants.
On top, the standard collared shirt will serve you well (not tucked in, of course!) You could mix that up by wearing a sweater overtop. You really only want to wear a t-shirt if you can dress it up with a blazer (see Orlando photo) or perhaps a zip-up sweater.
More tips found in Shoes, Pants, and Shirts
More event-specific tips: Beach, Formal Occassions and First Date
[Events]
"Keeping your clothes well pressed will keep you from looking hard pressed."
Coleman