1. Only use a belt to make an outfit interesting. There is a great danger with constantly adding a belt ( a la Gok) as a style statement which tips an look over the edge.
2. How do I know if I'm adding interest not going overboard? I still have a horror of the boho look that was the bane of my life circa 2003/4/5 when every woman alive seemed in possession of a belt with a big round metallic buckle for decoration. Usually worn over the hips of jeans or long boho skirt. It was the rise of the Ugg boot and the shaggy or faux fur gilet. Beloved by every retailer and the consumer - that belt transferred easily to the real and faux Juicy Couture track/leisure suit thing that was going on too. This is an example of a belt being overkill or overboard. It was essential to the look but did not add interest.
3. How to add interest? Take a shift dress which is tailored and has a defined waist -think Anna Wintour. Now our Anna is not a woman in search of a belt, instead she relies on her tennis in the morning and her coffee to keep her trim. Also Anna is quite prescriptive and her necklace(s) add to her signature style. If you have such a dress then you can add to it with either a necklace or a belt.
4. How do I do that with certainty? Use your eye and check your mood! Some days you might add a belt to the dress and it looks/feels great and on another you hate it. Your moods do have an effect on when you wear an item and not just your clothes. If it feels good do it but if not so what.
5. But I haven't got a shift dress. How else do I wear a belt? Okay here's a few scenarios! Long boyfriend cardi, top and skinny jeans. Add belt over the b/f cardi. Shirt and skirt - add waist belt at waist of skirt. Top plus black trousers - try a hip belt, these have a curve at the back of the belt. On no accounts wear a belt as described as above! Wear a belt over a jacket or coat. I was wearing a navy blue tunic dress with a black jacket the other day and wanted to look more formal so I did up my jacket (which was a charity shop find ages ago) and added a navy patent belt at the waist. I breathed in all day but it did the job. Look at old magazine images for a vintage take - remember those 60s dresses with a gold chain belt. The trick is not to be prescriptive or rule bound this should be experimental to develop your own sense of what suits you.
6. What about my body shape? Even though I know everyone is obsessive about body shapes I prefer the idea of proportions. One of my friends is an apple shape and sworn off all things round her middle because of her voluminous bust. But her back size and round her ribs under her bust is her tiniest point. At first I introduce a ribbon on her favourite empire line dress (which does not look like maternity wear due to gorgeous pleating flowing beneath) and then a very skinny belt. It isn't something she does a lot but occasionally it lifts her mood when wearing the same old dress. You just have to try differing widths of belts in different places with an item you wish to dress up. Therefore you go in said outfit to a variety of shops and try on lots of belts.
In short, experiment, use your eye and don't be afraid no one is going to appear with a big foam hand pointing out your belt wearing. I wouldn't do it every day, and go for weeks without donning one but is nice to have in your arsenal of style props.