Here's the Thornwolf way to keep your canine heads in perspective. If you've ever seen me draw you'll see that I pretty much keep to this formula once I'm starting out. Not exactly so rigid but its basically the same idea of starting with the head shape and the direction of the muzzle as a rectangle and viola.
Please remember this is how /I/ personally do it. The Thornwolf way is not always the /right/ way ;D *cough*yesitis*cough* >_>