The one exception is that black will always look good with black.
Most belts have a width of one to two inches, but the best belt for a casual look is usually going to fall between 1 1⁄4 and 1 3⁄4 inches. The thickness can vary as well, from about 1⁄8 to 3⁄4 inches. The best fitting belt will be long enough so that it can fit through the first loop after the buckle.
Although it is also important to veer away from a belt that is too long. A good rule is to keep the belt under three inches out from that first loop.
A casual belt can have whatever sort of buckle you prefer, small, large, shiny, single or double pronged are all good options. Although usually a D-ring buckle is the most common.
The basic rule for buckles is to keep it aigned with the formaity of your outfit.
There are plenty of other styles of a casual belt including canvas, webbed, needle point, and the critter belt. The latter two are associated with a preppy style.Braided belts are another option and can offer a nice texture.Belts follow the same style tips as any other article of clothing, less is more so be careful not to over do it with other accessories if you opt for an interesting belt.