a sister guide to my last head tutorial(http://www.furaffinity.net/view/27629445/) again I don't think this is gospel but it's definitely how I see things.
footnotes:
muscular cheeks and square muzzles are more common in solitary ambush predators, bone crushers, and large game hunters, and effects the whole skull with anchor points, and even enlarges the neck in many cases
with bears and mastiffs/pitbulls: don't afraid to add that lower lip/wrinkles! flabby faces add to the realism! smaller eyes help too.
Great danes can be extrapolated from this too. lengthen a heavily jowled muzzle and give it a slight'ridge' across the bridge of the nose with shallow cheeks.


Feel free to ask questions or inquire about future tutorials and guides!