Breathing exercises have worked well for me. The breathing technique BalishBun mentioned is a great one. It's a form of yoga breathing. If you Google "alternate nostril breathing" you can find out more about it.

Diaphragmatic breathing is another great technique that is very calming.

I was in group therapy about 13 years ago and something they taught us was to wear a rubber band around your wrist (not tight, just like a bangle bracelet). When you feel an attack coming on, snap the band, not so it hurts but so you can feel a little sting. Sometimes you can trick your mind into focusing on that sting rather than the anxiety/panic.