ONe thing that helped my dog on stays immensely was intense distraction by another person with me standing close.
For the sit, I would have the person come up and forcefully command DOWN, using body language, pointing, etc., with repeat commands if necessary.
I would then prevent the dog from downing, (using a buckle collar and leash) and each time the dog almost made a mistake, once they were sitting again, food reward and praise. After a couple of mistakes, I would have the distractor back off slightly so the dog could be completely successful.
This single exercise probably made my dog more solid on stays than any other. We also proofed with many other commands, including assuming heel position by another person, asking for the dog to heel off with them, etc.
The only rules were no touching, no release word commands, and no using the dog's name.
The dog gained an incredible amount of confidence on her stays using these methods.
She held a sitstay in the open ring while a male dachshund rushed up between her front legs to sniff her vulva, and then ran around her and put is paws on her back before the steward got to him.
I saw him run up between her front legs before we went behind the barrier.
The ring stewards were astounded that she didn't break.