I don't know why, but my kids do really well in this situation. We often say "maybe for your birthday" or "we'll add that to Santa's list" or "we're not buying toys today". They might make a little fuss, but so far it's been pretty easy. We also do the thing where you repeat back what they said to them, like "oh you want that don't you? I know, but not today".

Edit oh and they are pulled out of the store if they don't behave, so there's that too.