TIP #1: RATE OF REINFORCEMENT
Rate of reinforcement (RoR) is the number of rewards per amount of time. A high rate of reinforcement creates a snowball effect, making it easier and easier to succeed. Successful performance of a behavior becomes habit.
A high ROR adds value to behaviors and situations. You can make your dog really like to do a behavior (operant) or really like to be in a situation (classical) with free-flowing treats. Alternatively, a low RoR can reduce value to behaviors and situations especially when distractions are present. If your RoR drops too low you'll notice that your dog may "check out" or leave you. Zero reinforcement should extinguish behavior. It certainly will if you add value to an alternate or incompatible behavior.
TIP #2: NOT ALL GUARDING BEHAVIOR IS INAPPROPRIATE
Resource guarding is a natural, normal canine behavior. In fact it's a normal behavior for most warm-blooded animals. Even we humans guard our resources - sometimes quite fiercely. Think about it. We lock our doors. Store clerks have loaded .22 rifles under checkout counters, while homeowners keep shotguns and baseball bats leaning in the corner by the back door. Banks keep valuables in vaults. Some of us get insanely jealous if someone pays too much attention to our significant other.
Dogs guard their resources as well, sometimes quite fiercely. This is most troublesome when they guard from humans, but can also get them in hot water when they guard from other dogs. That said, some dog-dog guarding behavior is quite appropriate and acceptable.
As an example: In a dog park or doggie daycare, Dog A is chewing happily on a (insert any valuable resource here). Curious, Dog B approaches. Dog A gives Dog B "the look." Dog B quickly defers, saying "Oh, excuse me!" by calmly turning and walking away. No harm done. Much of the time the dogs' owner isn't even aware that this occurred.
- Whole Dog Journal
Jeff Dentler, CPDT-KA, IAABC-ADT, FFCP, CTDI