Avoiding Tantrums at the Store

Do your kids drive you crazy asking you to buy them things while you're at the store with them? They start by asking for something they see, but as soon as you say "no" they begin to escalate the situation with pleading, demanding, and sometimes an all out temper tantrum. You try not to over react, but it seems like the only way to get them to stop is to give in and buy them what they wanted,