The right belt can be the difference between a decent outfit and a great one.
1. Tops: Don't belt a top. Cardigans, tunics, blouses, T-shirts, or sweaters you choose to leave untucked do not need to be belted — belting them ages you, and more often than not, it just looks awkward. If you want to emphasize your waist in a certain top, tuck it into a skirt and then belt it.
You want to believe an outfit like this will work in real life, but that's just the artfully disheveled hair and perfect studio lighting working their magic spell.
2. Jeans and Pants: If your pants rest on your hips and they don't have belt loops, don't wear a belt. It's going to slip and you'll spend the whole day trying unsuccessfully to keep it centered on your waistband.
3. Dresses: Belts can and should be used to add a little extra shape and visual interest to dresses, but you need to be careful if you're trying to create shape where there is none. Using a belt to create a waist means bunching a lot of fabric together, which changes the way the dress drapes and can make it start to look sloppy.
4. Skirts: Belts look great on high-waist skirts. Just make sure the middle of the skirt band is resting snugly on your natural waist so the belt doesn't slide around.
5. Coats: If a coat comes with its own belt, use it. Don't try replacing it with a belt of your own. Fashion magazines try it from time to time, but it always ends up looking labored and conspicuous. You want your outfit to feel easy and natural.
Any belt other than this one would have different colored hardware or a different texture than the rest of the coat and be distracting, rather than pulling the whole look together.