When I am out cruising photography message boards for people looking for CC (constructive criticism) one of the things I am always looking for in portrait images is if they got light into their subjects eyes. When I first started out I had no idea about this, but once I learned it, there was a huge difference in the look of my portraits.
What does it look like when you don't have light in the eyes? Here is an example of Mike from my brother in law's wedding a couple years ago that I took
you can see that his eyes are just flat and dull (maybe not in the small size, but here is the larger image.) Don't judge me on the lack of sharpness, that was taken well over two years ago. If you look closely you can figure out that it was a very overcast day, and the sun was pretty much overhead so the direction of the light on them is down, not into the eyes.
What does it look like when you do get light in your subjects eyes? Note this doesn't just apply to people, but also to animals, you can even to an extent apply it to flower macros as well.
This was also an overcast day, and the sun was overhead, but by having her look up at me, the light was into the eyes.
This one is a bit more of a snapshot of my two best friends, but you can see how great their eyes look with all the light getting into them:
Side note, digging through all these old photos for examples is really driving home to me how much my photography has improved over the years.
Now that you see how much good this does for your images you are probably wondering how to do it yItourself? You have several options, like everything with photography, one is to use natural light like a window if you are indoors or daylight if you are outside, or a flash or even a reflector with any of those light sources. For now I will focus on indoor window light since that is what I have the most examples of and I think it is the easiest to grasp. So first, no flash!
After that it is pretty easy, get your subject to look towards the light source. Done.
See the window at the left, and Stanley looking at it?
They don't have to look directly at it to get the light in.
If you get between your subject and the light source you will get good catchlights as well
Now go look for your light source and get your subject to look at it.