The most important thing to do during a panic attack is to think positive thoughts and tell yourself that your going to be OK, even if you don't believe in them! Example:
"I am going to be all right. These feeling are not rationale. I am just going to relax, calm down, and everything will be all right. "
"These feelings are not dangerous, they are just uncomfortable. I am fine. I will continue with what I am doing or find something better to do."
If your finding it difficult to think positive thoughts or to concentrate on relaxation strategies, find ways to distract yourself. Some people calm down by redirecting their thoughts to...say...the pills they are carrying! They start obsessing over these pills instead of their fears. They get into an internal conflict over whether they should take the pill or not. They keep weighing the pros and cons of taking one, and miraculously, their panic attack just fade away without them noticing.
The important point is to shift your attention away from anxious thoughts.
If you have not been doing so, start exercising now as they really do make a HUGE difference with panic attack. Swimming, cycling, jogging, brisk walking, etc...these are all simple exercises that anyone can do. You might want to consider taking up Yoga or Pilate classes as you can really learn various useful breathing technique to calm yourself. Additionally, they strengthen you physically and mentally.
Lastly, its important to start accepting panic attack and accept the way your feeling. Consider this fact, do you spend more time worrying about having a panic attack than the time you spend having them? Won't it be MUCH better if you just accepted these feeling? This way, you will feel less angry and frustrated about it.
Use your time to bask in calmness instead of worrying about another panic attack, this is the KEY to stop panic attack forever!