Airing-Literally pumping air under the skin to create fill(I've also heard of people using oil instead of air).
Pumping-Shoving a tube down the calf's throat and filling it with some kind of fluid(Sprite, water, electrolytes, etc...)
Twinning-Most shows outlaw this, but it still happens from show to show.
Painting-Some breeds have rules, some don't, some shows outlaw it, some don't.
Back dating-Not registering the calf until its been on the ground a month or two, so it's got a size advantage.
Drugging-Too many to list. There are steroids, anti-inflammatorys, anesthetics, etc... Most commonly used to battle soundness or disposition issues.