Dealing with panic attacks is no fun. I've had to do it myself, sometimes on the fly. These techniques were developed mostly without any premeditation. The only meditation that went into this writeup was how to present the information. All right, let's roll.

1) Get some fresh air. If like me, you're stuck inside, open a window, and sit near it. Just let the fresh air caress your face, and breathe deeply. No matter how strong the temptation to hyperventilate, try not to. It doesn't help, and can make you more panicky over time.
2) Do energy healing on yourself. This takes a strong imagination. If you're familiar with grounding and centering, great. For those who aren't, here's a crash course.
A. Imagine all the panic that is centered in your chest or belly as icky black gunk, sludge, etc. Now, push it all down through your feet and deep, deep into the ground.
B. At the same time, draw in brilliantly white light in from the sky, filling yourself, packing it down into your body like filling a tube with toothpaste. Pack it in tight.
C. If you, like me, are very strong in the technique of energy healing, imagine injecting yourself with Valium, Ativan, etc. To do this, do the following:

Imagine that your dominant hand, the one you write with, is full of the molecule. Now, think of where a vein is. If you've had blood drawn, remember where it was taken from. Place your pointer finger over the spot, and press it lightly against your skin. Imagine the substance which is really energy flowing into your body and taking its effects. This can also be done for cronic pain, insomnia or in any other situation where the symptoms do not require emergent medical intervention, but you don't have the medication on hand. This works best if you know the effects and can visualize them taking effect, and fast.
3) If you believe in prayer, do so now. Ask for strength, tranquility, etc.
4) If you believe in repetitive prayer, chanting, etc, you can do so. Use a set of beads, knots on a string, et, something that is damn near [indestructible. I would keep such a device on you at all times. I carry my mom's rosary on me, and use it liberally. I couldn't give a damn who is around. If someone is offended, that's their fault, not mine. I'm not saying I don't care about another person's feelings, I do care. But when I'm in the throes of a nasty, evil panic attack, I'll do what helps me, and counting on my fingers isn't an option sometimes.
A note on this option. Be absolutely sure the prayers are firmly cemented in your head. Trying to leaf through a book is distressing enough, and much worse under pressure. Don't put yourself through the strain. Learn the petitions well in advance of actually needing them.
5) Run your hands under hot water. As hot as you can stand. It helps by lowering your blood pressure by causing vasodilation, even if only temporarily.
6) If you smoke, go for it. The main goal here is distraction, not to get whacked out on nicotine, so one or two should do the trick. Don't go overboard.
7) Chamomile, ginger, valerian are all good ideas. If you can do them, and if they will not make you unconscious, like they do my husband, go for it. Valerian root comes in capsule form, and in tea form with other herbs. It also comes in a tincture as the pure root form, but that's somewhat hard to find.
8) If all else fails, and if your doctor has prescribed them, take the medicines you were given. I note this down last because it is the most extreme measure, and the one that, while working for the longest, may in some cases only mask the symptoms. Besides, in some places, carrying the medications without an ID, such as a driver's license or state id card may be illegal. Carrying herbs, prayer beads, etc, isn't.
One last note. If you do so, you can write in your journal. It has helped me over the years, especially with my nearly uncontrollable [temper. Even just writing down stream of consciousness thoughts, even if they don't make much sense later, can help to alleviate the crushing panicky urgency of the moment. Remember, write this down in a quick reference form, something that you can refer to in a hurry. I know I'm being wordy, but once you've practiced the techniques, especially the energy techniques, they will become second nature and you'll have no need to think them through. Also, there is no need to go in the order I wrote them. You can proceed in any order you need to. If taking your meds if you preferred route, then go right ahead. If herbs are your best shot, be my guest. I usually breathe deeply first, then pray, then run my hands under hot water, then smoke, then do energy work, then, and only then, meds. Hang in there, and it will all be ok in the end.