I don't know that it is totally possible, but I think if you ignore it, you can reduce the length of time that they last and ultimately reduce the number of times they happen. They want attention. My daughter threw a big tantrum at American Kids last week. I just laid her on the floor a few feet away and gave my other child all the attention. I told her she could join us when she was done. Then I apologized to the parents nearby! It's just part of childhood.
Hmm not sure. I know when my son who is 22 months does that, I just ignore him. When my hubby's daughter did it, I didn't have kids at the time, but we would just leave. She would always do it when we pass the toys part or when she doesn't get things, but then she realized that with her throwing fits she isn't going to get a thing, so it eventually just stopped.
Oh I remember with my mom, she has 5 kids, so she would reward us. Basically, if we went the entire time in the store w/o complaining, talking back, getting into trouble, or asking for things, then at checkout, we get something of our choice. It was always at the end too. Try rewarding. It obviously doesn't have to do with candy or anything, but maybe something cheap like a traveling color book or something. Something that will give them something to look forward to, you know?
My son has tantrums all the time. I have started making him sit in the basket when he starts acting up. He is almost 7 so he hates the basket.(take 2, hubby pushes one). He will calm down and politley ask if he can get out. Then, if he acts up again, daddy takes him out to the car to wait for me to finish up. I tried the rewarding thing and that just made him think that each and everytime we went to the store, he got a prize. And that was not good.