A good rule of thumb is to make sure the belt has a purpose beyond mere decoration.An example of deliberate belt usage is when it alters an existing shape as is generally the case when you have loose fitting garments you want to cinch in. See my [very] rough sketches below:
Of course, belts don't always necessarily have to create shape. Sometimes, the shape is already there and the belt steps in as a useful accessory to divide a silhouette and separate top from bottom. This is generally true when you are tucking a top into a bottom. After all, there is a reason why trousers have belt loops even when you can wear them without fear of dropping your pants sans belt. Again, see my rough sketches below:

The usual way in which women go wrong when donning a belt is when the final shape created is unchanged and somehow the belt actually draws negative attention to an unflattering area. See the example below taken discreetly with my mobile:

Now before you go ballistic and charge me with some pejoratives about my "weight-ism", let me just say that I am in no way criticizing her figure--just her outfit choices for her figure! Instead of the skin-tight ensemble, she would be much better served in something a size or two larger, more comfortably-fitted, and then possibly belted. Might I suggest a casual shirtdress with an A-line bottom plus belt? Also, even if she were stick-thin, this use of a belt would still be unacceptable.
Finally, I have one more example of an instance where a belt appears to be thrown on for no real purpose. I'll leave you to read my explanation below:

All right. That's all folks! Happy belting!












































FOLLOW ME
I had set my ITunes to shuffle and suddenly this song came on: “Superman” by Eminem. In listening to the lyrics of this song I hadn’t heard in years and years, I was really, well, shocked.
I mean Eminem is no stranger to the criticisms of feminists the world over, but how is it that the public as a whole accepts his clear, unabashed misogyny? Here are a couple of choice lines:
Meh. I could write an entire thesis but I'll just summarize and say that it seems true that everyone, men and even women, have a little streak of misogyny running through them. As a woman, I can hear those lines Eminem’s spouting and explain to myself, “That’s ok—he’s means that type of woman; that ‘muthafuckin slut’ type of woman”, and as a result, I throw my fellow sisters under the bus instead of rushing to their defense. Why do I do this? Because at the end of the day, it's a man's approval I seek. As long as I prove to a man that I'm not a "muthafuckin' slut", I'm safe.
We also allow Eminem his two cents here because despite his hate of his mother and his estranged wife (or are they back together?), he does have a daughter he truly seems to love and appreciate, pudenda and all. That means, he really doesn't hate all women--just the "muthafuckin' sluts". Right? Right???
Of course, what really is the distinction between the daughter, wife, sister, mother you love and the muthafuckin' slut you hate? And it’s not just the men who categorize; women will categorize each other just as quickly as a man does—to our own detriment, ultimately. Admittedly, I am guilty of this activity.
Ok, I’m not sure where this is going, but I guess all I can do—and hope that you do—is to be aware of how extremely sexist the world is still. Don’t scoff when presented with those statistics about how women earn $0.70 to a man’s $1.00; it’s not about money. It’s about how despite everything, we’re still sexist as all hell—sexist enough to have lined Eminem’s coffers and laugh at his jokes and hardly bat an eyelash.
I apologize as I didn't mean for this to be some sort of fingerpointing session at Eminem whose latest song (while also tinged with sexism) is quite catchy. It was simply what I happened to be listening to but he is far from the only artist/television personality/politician/etc. to have marginalized women. Society's acceptance of him and the rest of it is telling of society's acceptance of sexism as a whole. The eradication of racism may be progressing in leaps and bounds, but I'm sad to say that sexism is still alive and well.