Well, there's a lot to be said about one of the hardest things to tackle -- portraiture. For starters, I wanted to mention the rule of thirds. If you are trying to be proportionally accurate -- (and feel free to absolutely not strive for this) -- then it helps to draw faint lines to the side, dividing up the egg-shaped head into thirds. Eyebrows generally reside around the first line, with eyes actually resting close to the halfway mark. Lips, especially the lower one, tend to rest on the second line. And ears...notoriously hard to draw without them looking like some kind of alien species clinging to the sides of the cheeks....are positioned such that the top of the ear corresponds with the edge of the eye, with the lower ear lobe lining up with the upper lip. That last bit has helped me loads, over the years, since I have a tendency to draw ears either too small, or too high on the head...checking alignment relative to eyes and lips helps me correct.
More tips coming, in another 2 weeks!