I would also like to recommend Betty Edwards' "Drawing on the Right Side of the Brain," where I sourced a couple of the diagrams below.
Of course the best thing is to rely on your observations, but these diagrams should help you get started with portrait drawing.
5 Useful Tips for Drawing Accurate Portraits
1) The most important ratio to remember is that the eyes are in the middle of the head. There is a tendency to want to put the eyes towards the top of the head, but this is incorrect.
|Facial Proportions for portrait drawing|
|Structure for starting a 3/4 view portrait|
|Cut-off Skull Error|
source: Drawing on the Right side of the Brain by Betty Edwards
|Use sighting technique to measure the angle from nose to chin.|