I'm at about the same stage as you, still early days for me and I don't have a lot of time to practice painting but what I've found really helps is painting under a magnifying lamp like this:
It helps to see the finer details and makes the end result look much cleaner (but then again maybe my eyes are just bad).
Use thin coats of paint on the hair and eyebrows to fade the transitions.. I have trouble here but generally painting on individual strands of hair in different shades of the hair colour works pretty well to add depth around the hairline and for the eyebrows.
Vary your skin tones a bit - you want to paint thin, using a variety of similar shades.. (i.e. start with a base skin tone and then mix in other colours to add shading in areas that need it on your successive coats).
Use some very watered down red to flush the skin and add mottling, and use a dark pink of some kind to shade the areas around the eyelids a little to add some depth and contrast there too.
Putting into practice is hard, but the above tips have helped me to slowly improve the end result each time over the last few that I've done. The Jack Bauer one is the last head that I painted, and was basically a test to see how the skin would look using the red shading + mottling.
