Learning to portrait:
1) Get some tutorials. There are a ton of books, articles, and write-ups on how to draw faces and heads. Several are linked in the resource thread here on Pixelation. They all offer similar advice, guidelines, rules for proportions and placement. Later you will want to break these rules, but you'll need to learn them first.
Grab 2-3 different tutorials. Enough to experiment with different proportions, but not so many that it is overwhelming. Take an hour or two and practice what they teach. Use a simple grayscale, 5-7 colors, for the moment. Also, most of these are written for drawing with pencil - thin lines of infinite resolution, while pixels work with large colored blocks. So it will be a bit different. Work medium sized, like what you've got here.
2) Taking what you looked at in step 1, study some real faces. Friends, neighbors, photos. Compare proportions, angles, positioning. I find it best to look at group photos. They give you a variety of faces at about the same scale and lighting, which makes them easy to compare. Avoid pictures of celebrities and models because they have been airbrushed or photoshopped.
Pick out two or three nice faces and pixel them. Go large with this, about 64 x 64, gray scale again (quicker). Don't spend more than a couple of hours on this. If you're having trouble figuring out whether to go lighter or darker for a particular pixel, take a look at Helm's article on pixel clusters and banding. It helps. You may learn that facial shapes don't always look like you think they look. Nostrils, for example, are not just round holes.
Here is a sample group photo. Cheesecake, but SFW. http://www.4freeimagehost.com/uploads/934ca6ebbb19.jpg
All slightly different angles and shapes. Or use your own. Study this only after reading about facial structure and proportions or the average face in step 1.
3) Eyes. You can do this step before, after, or along side step 2. Eyes are actually a lot smaller in real life than you expect because your brain places such importance on them (results from step 2). I like eyes to be very clearly defined in my pixel work, and go a little larger than the average sizes per the tutorials in step 1. The dollerz community spends quite a bit of effort on eyes, so I recommend studying what they have done.
There are a ton of dollz sites, but here's a good starting point for eyes: http://xandorra.bluedaffodil.net/tut_eyes.php
4) Another topic to study might be cartooning. When you're ready to start breaking the rules in order to draw the face you want, follow the trails blazed by cartoonists.
For this particular face, the eyes and nose can come down a pixel. Noses are a little triangular when viewed from the front, Places where the face dips in (eye sockets, bridge of the nose) are generally a little darker. Ears are a little high.
Here is my edit. I replaced one of the lightest colors with a desaturated pink for the chest. I'm still a bit weak on hair.
I stuck with heavier outlines to be consistent with the large dark border. I think it works out well. There is a bit of banding on the corner of the chin, but otherwise I'm pleased with it. No reference used.