After you've recognized the attack, begin to count in your head. Count 1,2,3,4 -- 1,2,3,4 -- 1,2,3,4 pausing on the dashes and so on. Now you might be thinking:
"Hey, that's easier said than done. When I'm having an attack I can't even think."
That's why you're not going to say or even think the numbers. You're going to breath the numbers.
Take a deep breath in through your nose for the 1 and 2. Then take a deep breath out through your mouth for the 3 and 4. Practice this routine for a few minutes a day until it becomes an automated exercise for you.
Practicing really helps a lot. If you practice in a controlled environment, when a Panic Attack hits, this self-defense mechanism will seem routine.