I want to improve portraits

[BrushNoir]

Sure structure is useful, I'd say get to know the asaro head then, start by the simplified one and move your way up to the more advanced one. It seems like ssiddiqi has some sort of understanding of placement if you check the thirds. Studying asaro or reilly gives you an easier measurement system. In my opinion, reilly is not only for tieing things together, it's more like an roadmap which tells you where to place your features and how big, as I said, it's a kind of measurement system.

Here is a rough sketchover based on the first drawing. I fixed the ''proportions" a bit using this method.







Sure you can do self portrait studies and master studies, but for me it helped to have a general head as a basis to start of. So yeah Asaro + reilly still :p 

Check out http://deadoftheday.blogspot.nl/p/downloads.html the head section to get you going for improving head drawings.