
I'm not great at explaining heads.. The link Westcoast has given actually is a couple of show people.
One inportant thing for heads is to strip them a little but often, it gives the volume. If you dont know how to strip a head properly though get someone to show you as you don't want to be yanking a dogs hair. Just pull/strip a few hairs here and there with chalk will give great volume.
The head hair although when brushed into place doesn't look short actually should be kept quite short. I think leaving it too long is a common mistake by groomers, it ends up in the "pet" middle parting. I would say sort of 2.5 inches. ( I need to go check a dog ;-) )
I actually comb it up through my two fingers just like a hair dresser does for both thinning scissoring and stripping the head.
If you brush the hair all back sort of flattish and create your "circle" with thinning scissors ( not scissors) whilst in that position it works better. Also if you have an imaginary line from ear to ear the hair should be short from that line back down the neck. Another mistake I often see when there is too much hair extending down the neck. I often use a hair dressers clip to make my line seperation.
Thinning scissor the bridge of the nose into the stop but not up above the eyes too much. You don't want the pokey face look.
Not sure how much sense these tips make as others have said if you can get 20 mins with a show person in person that is your best bet.
If you ask most people it is the heads that are the hardest to get perfect.
Good luck. :-)