In addition to working with a professional, I really like Jean Donaldson's book, Mine! A Practical Guide to Resource Guarding in Dogs
In my experience, teaching dogs that giving you high value items is very rewarding for them is the best way to go, because when it comes down to a very bad situation you want them to be able to relinquish an item without thinking about it. I've had a couple of situations where I've had dogs "drop it" on instinct because we've worked on that principle...that "dropping it" and giving me things is really rewarding. Our previous girl, Shanoa, dropped a live rabbit out of her mouth that she caught in the yard, because we'd worked on drop it very often and it was very rewarding for her to drop stuff for me. Richter will drop and relinquish his raw bone when I ask, which is very high value for him.
I don't ask my dogs to give up good stuff often - I teach drop it by always giving them better things when they give me something good, or giving back the item they had, so they learn that giving me the good thing doesn't mean they will lose it forever or that they will get something even better. They learn they don't need to "protect" stuff from me. When I put my hand near their food bowls, I don't take stuff out or stop them from eating, but I drop in really yummy stuff, so that a hand near the food bowl means something awesome is about to happen. I also often just leave a dog alone with a really good chew thing (like a bully stick) in their crate until they finish it...when I have a super yummy piece of pie I probably wouldn't want someone sticking their fork in there and stealing my pie, LOL
We don't always need to mess with dogs when they are eating.
If you teach dogs that they will be left alone, or, that they can trade you for better stuff, you can teach a really solid drop it because they are eager to do so, knowing that they don't need to guard from you - you are the giver of great things! It takes time to build this foundation, but it's very worth it. Some day when your dog picks up something dangerous you don't want to have to pry open their mouth (or, if you do need to examine your dog's mouth, which we all should, on a regular basis, you don't want a dog that is worried or aggressive when you do!).
Just my two cents and the way I've trained my dogs successfully to not resource guard from me. Resource guarding BETWEEN dogs is a different thing all together, and Jean Donaldson's book addresses that, too, but I won't get into that here.