Temper Tantrums

I have discovered I have no idea how to deal with a raging temper tantrum. My middle child is in the midst of a full fledged on the floor kicking the feet temper tantrum. I have tried timeouts, ignoring, grounding...to no avail. The funny thing is, I never give in to a temper tantrum. The rule is "Throw a fit and you have just guaranteed you will never get what you want." And yet, the temper tantrums continue. I am bewildered.