Wonderful question, Voltaire. The way to deal with territorial over things, is to give dog his/her own space in which he has a place to be with toys, etc. where no one else is allowed access. It is good to teach a dog the command "drop it" and have him give you the toy. Then be all happy and return to him saying "here". This establishes your leadership and that while, you might want something, he will get it returned or something wonderful in return. Also never play tug-o-war with your dog. This is a game that a pack leader, in a dog pack, never, ever engages in with other dogs. This is a game played by puppies and secondary dogs. These are things I have only recently learned in the past few years thru a friend of mine who is a well known canine behaviorist and master dog trainer, goes to show you certainly never know everything in life.
Something I use with my dogs is practiced during feeding time. The command to eat is also "here" this means it is now yours not mine. "mine" is the command I use to say this is not for you! I put the dogs dish down and say "here" then I will say "wait" and briefly pick dish back up and say "good wait" and quickly return with a "here" command. I also use a lot of brief strokes and touches on the dogs back while they are eating. I have never had a problem in 26 years of boxers with any territorial growling over anything where I am involved!
Sometimes there is some amongst themselves, but since I have established that I am in control of the good stuff, I can remove the item at any time and redistribute or put away. This I would like to stress, I HAVE STARTED WITH YOUNG PUPPIES, not a dog with a current problem. It you have a problem with this I would consult a professional trainer. But the "here" "drop" and "mine" commands are indespensable in our household to establish my control of the good stuff.
My last female did a warning growl, snap, at a toddler who poked her finger into her eye when she was cornered while laying on her dog bed. I could not punish for this warning and merely removed the dog to her safe space away from the offending child. The parents were completely understanding of what had happened and the child was unfazed.
Nano