I completely agree with NittanyEquine.
One thing that's helped me get a super "whoa" on Lacey is saying "whoa" (actually "ho" in her case) and backing her up 6-10 steps. She caught on quite quickly that if she stopped when I said "ho," she wouldn't have to back up, but that if I said "ho" and got no response, well I'd be up in her face making her stop AND back up. Haha
And what NittanyEquine described about the huge whoa has actually happened to me. Hahaha I trained Lacey to stop really well but I hadn't cantered her yet with this new knowledge of hers. So we were cantering along and just since "whoa" had been so ingrained in my mind, I said "ho" and boy did I realize the error of my ways!
She planted her feet, scooted her butt under herself, stopped dead, and then jumped forward to keep me in the saddle. It was one of the funniest moments of my life, she turned her head and looked at me like "you are a fool, a certifiable fool" (to make matters worse, we had an "audience" of people who saw me riding and starting grouping around the arena, basically "Hi guyzzz! I iz REEEEAAAALLL good riderz, pleeeze watchez me make a foolz of meselfz?" sorta situation only minus me toting myself as a good rider, heehee).
After that I taught her "easy." To her it means to go slower while staying in a walk/trot and if she's cantering, it means trot. So now, I say "easy" to get a slower gait, and then I say "ho."