I simply stack them daily from very early age. I also teach them the command "stand".
If they wriggle, pull their feet etc, I say "no" and put the feet back where they are supposed to be. Once the dog is standing properly I praise them and let them go
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When my dogs wriggle on the grooming table, they get told, BIG TIME. When they are puppies, I put them on the table every day, and "groom" them (mostly just pretend to do so). They have all learned sooner or later (some need more "telling" than others
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When grooming other peoples dogs that are not that good at standing, and pull their feet all the time, i simply lift the other back/front foot up (the one i am not doing at the moment). It stops the pulling all together, as it's very hard to pull feet without losing balance, when one foot is in the air
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