There are many guidelines, rules and assertions about lighting, but I've come down to a few which I instinctively keep to when lighting my photo shoots. Take these with a grain of salt as these pertain to my style.
One Key Light
This is more a rule than a guideline in my mind. The idea is that you only use one of your light sources as the light that defines the shape of the subject's face and body. All other sources only aid the composition and don't try to compete with the key light.
Even when you are shooting on a cloudy day, it is still one light source. Though extremely soft, it is generally coming from above. The Sun is the one source of natural light, so that idea lends itself to manufactured lighting as well.
As you can see, there is one shape defining light (the key light). All other sources of light are less powerful and are often acting only as fill light to bring the shadows up and reduce the overall contrast. The third being the softest and only lit by bright, full window light bouncing around and providing substantial fill.
Key Lighting Tends To Come From Above
The Sun follows an arc above our heads from East to West. As a general rule of thumb, it doesn't tend to look nice when the key light is coming from under the face unless that is the effect you are pushing for. Horror and suspense often utilises this "abnormal" lighting to great effect.
I've found that if you are having trouble flattering your subject from a side angle, moving that light above and center over your subject will often change the portrait entirely. People are used to seeing light from above so this tends to be why it works well.
Contrasty Bright Rim Lighting Adds "Glam"
You can easily add a "glam" factor by making your rim lighting less subdued. As you can see in the two photos below, the first has strong rim lighting whereas the second does not. The second looks more natural and the first has a staged, studio feel.
Both the contrast in the scene and the relative hardness of the light sources helps to separate them into different moods and styles.
These are some basic tips about portrait lighting. Things you learn in lighting subjects as well as the reasons why they work.