This is the physical handling "protocol" we teach for increased predictably at the vet, groomer, etc.
As you handle each body part, you are feeling for resistance. You do not move on, or stop the exercise, until you feel relaxation. It’s best to go through in a predictable order. We go over every part of the dog. Generally a dog will have four feet and legs, a tail, a belly, a back, a head, eyes, ears, nose, and mouth. Don’t miss spots. Definitely don’t skip the parts they have issues with. The whole point of this exercise is to work through those issues.
The dog is rewarded by ending the exercise. For most adult dogs, practiced already in objecting to handling, that is the biggest reward to them anyway.
If needed, one person holds the collar and another performs the handling. A proper collar hold and arm perpendicular to the dog's neck puts you behind the sharp parts. A safety muzzle (leash around face) puts your hand behind the head, still away from the sharp parts. If you have to do that, this is probably still a 2 person job.
Over time, the collar hold creates a conditioned response of relaxing (if the rest of the exercise is performed correctly).
You screw this exercise up by letting go or stopping at the wrong times. By doing the handing in a forceful way. By being anything but emotionally neutral.
If you need to, muzzle or safety muzzle. If the dog is big, you build scaffolding to further restrict movement while working this exercise, and do enough training prior that doing so isn't a ridiculous proposition. If you have a dog that will end up very large, don’t be foolish and start this as a puppy so it's a non-issue by adulthood.
I don't use food or any other extrinsic reward until we're well practiced, and then only when the exercise is completely over. I don't want to condition arousal as part of this picture. This is also the reason you want to find a vet and techs who can be quiet and just do their work. They are not paid to try to make friends (and actually make things worse for the dogs who have trouble with this)… they are paid to do their job, which is rendering medical care.
Once this is taught, the collar hold is used for whatever procedures are to be done, until the dog has enough self control and obedience that they aren't necessary.