Suprisingly, though I do not believe this solves every fashion error, this is a brilliant rule-of-thumb to keep in your back pocket, if it's not there already.
If it's above the knee, wear flats. If it falls below the knee, wear heels.
Simple, right? When I first read it I felt a bit cheated. That's it? That's just common sense. But it makes those unsure circumstances that little bit easier when you have guidelines to go by: is it too short for heels? Well where does it fall?
One more thing: rules - especially in fashion - are there to be learned, understood, and then broken. This isn't the end-all-be-all of fashion; but it's a handy principle to keep tabs on, especially when going for that effortless chic, that I-barely-tried look that is French fashion. Use or abuse it, it's a good one to know.