When I was out and about, I wore what became known as my "uniform": dark grey or black sportscoat (three buttons, and NOT a freaking blazer, thank you very much), slightly lighter grey (if worn with black) or black (if worn with grey) dress trousers (with cuffs, thank you very much indeed). Cap-toed walking shoes - always black.
White oxford shirt, most of the time, though I'd mix it up sometimes and wear an arrow or tab collar shirt, depending on the tie.
Generally wore a red tie, although sometimes a green or blue one (go to Brooks Brothers or, if there's one by you, a Nordstrom's or Neiman-Marcus, they can help you with the tie - the tie is crucial!). For the pocket square, always white - starched and pressed linen in the warmer months, silk in the cooler weather.
This outfit opened doors that would have been slammed in my face otherwise.