I'm a guy that used to struggle with the being over- or under-dressed, but I've figured out a pretty easy system:
1) Casual (around the house, fishing, out running errands): Jeans or Khaki cargo-pants with t-shirt or polo shirt.
2) A bit less casual (Dinner with the in-laws, the evening service at my church, work): Black or Khaki slacks (like Dockers), polo shirt or casual sweater.
3) Somewhat formal (morning service at my church, clients at work): Black slacks or the pants from my suit (no suspenders), shirt & tie (I have a half dozen shirts and about 10 different ties). No jacket unless it's cold out.
4) Formal: Suit pants with suspenders, Shirt & Tie, Jacket.
I disagree with Msulinski about black. A black suit paired with shirts and ties in a variety of colors can be very flexible and appropriate for a wide variety of situations and environments.