The thing is with a head wip Claudio its very hard to crit in the initial stages because there are a hundred and one things to mention.But Tims {Dalifan} suggestion of the Messie tute is a very good one 
That triangle at the back of your head will stop you being able to get a rounded shape from an aerial view,as you need a 4 sided poly there to to be able to shape the back of the head properly.
I often use fashion mags from where ever I get them,hey have heaps of close up facial shots etc.
Also check out third_eyes brilliant head as well which has a thread in here.
Stu.