Make sure to have your dog around other many people and dogs at a young age (prior to 3 months) in a variety of circumstances.  Continue this throughout adolescence.  This includes having people and other dogs approach you and your dog out in public and also people and dogs visiting your home and property.  This is of particular importance for dogs who are guarding and mastiff breeds, and also those originally bred for watch-dog type behaviors.  It can also be helpful to teach your dog specifically what you would like him to do in those scenarios  (e.g. sit and stay by your side) so he has a "job".

Begin working with your puppy (and continue through adulthood) on teaching your dog to willingly give up items by counter-conditioning a feeling of value for doing so.  Start with items of low value and progress to high value items.  I like to use one of two methods: point to the floor with a treat in hand and when the dog drops the item he gets a treat; OR, the Chirag Patel method where we say "drop it" and toss treats to the side so the dog turns away from the treasured item.  Use a treat commensurate with what you are asking him to give up.  A dry dog biscuit may not be worth it if being asked to give up something special.  Try meats, chicken or cheese for difficult items. 

Also work on teaching your dog to leave things of interest and delayed rewards by teaching leave it (we use a version of Susan Garrett's "It's Yer Choice")

Teach dogs from the moment you get them as puppies that there is always more coming -- a treat for a sibling dog is a predictor that a treat will soon come to them.  I have the household dogs sit in front of me (only if non-aggressive), and keep offering treats to one and then another and mixing up the pattern.  They quickly begin to figure out they do not have to try to steal the other one's treat.  If the sibling gets a treat, he will also get a treat quickly after.  

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Don't feed your dogs in separate rooms unless there is already a history of aggression.  Don't put your hands in your dog's food bowls or otherwise bother him.  Instead, teach your dogs to respect other dogs' food and space by having them eat meals in the same room, but separated a distance from other non-aggressive dogs.  Use baby gates so they can see each other.  When the dogs each finish their meals, continue to drop additional pieces of kibble (or treats) in their bowl from time to time until all the dogs are finished.  This teaches them to stay near their own bowl and not to go looking for more from other dogs' bowls.  Slowly fade out the kibble dropping over the weeks.

Dogs have little control over their environment and their access to resources.  We decide when they are fed, when they go on walks, when they see friends, when they get special chews, etc.   Predictable routines help them to cope with that lack of autonomy because they know if XYZ happens at certain times of the day that they will soon be fed, or that they will be going for a walk, etc.  Regular deviations from routines can shatter their sense of predictability and security over whether their needs will be met during the day.  

You can start having your dog sit for everything wants (food, every throw during fetch or tug, to go inside/outside, for leashing, etc) so he learns that he has a behavior of value to offer when he wants something.  Use treats at first if you need to and quickly fade them out.  Nothing good happens without a dog's cooperation!  The objective is that he learns that he doesn't need to guard things.  Instead, we can use this protocol to create a give/take relationship where dogs are empowered because they have a way to influence outcomes.  They have a desirable way to say "please" and ask for something when they want it.

Do not leave out toys, chews, or other things your dog might guard.  If he is guarding the couch, the bed, or another spot, then use gating, laundry baskets or boxes to block access to it.  Feed in separate rooms if there is food aggression.  Keep your dog crated or on leash as needed.  Don't allow your dog to practice behaviors that you do not want to see.  After all, practice makes perfect!

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Meet with a professional (CDBC, CBCC-KA, CPDT-KA, IAABC-ADT) to help come up with a plan to address the issues.  Frequently, we are able to work through many of these issues with consistent training.