Let's start with a blank slate. Reasonable height for your respective age, broad shoulders with a taper down to the waist. This is what likely leaps to your mind when you think of the “average American build.” Shoulders are broader than the midsection, and the legs are lean, but still with definition. While we can't speak for every guy (literally, every body is different), you're likely the type that hits the gym occasionally, and aren't bulking up. Muscles are defined, but not overtly athletic.
When it comes to dressing your body, you probably have the easier time than others. Basically, there's a natural visual balance between your upper body and your waist, so wearing something slightly oversized (like drapey shirt) or something slightly skinny (like stacked slim jeans) aren't going to mess up your visual proportions too much. Unlike the other general body shapes on this list, the “trapezoid shaped” guy is also quite flexible when it comes to wearing prints. While there's usually general rules regarding what body types should avoid horizontal or vertical stripes, this shape has the visual balance to pull off either while not appearing stretched out (in the case of vertical stripes) or widened (in the case of horizontal stripes).
More to the point, you have slight definition, so emphasize it to make you look lean and athletic.
If you're dressing up for formal events, keep your trousers tapered, and your jackets too. This will emphasize the outline of your shape in a positive way, without leaving you with a billowing bottom half, or busting out of the lapels of your jacket.