I use breathing exercises, such as the following.
Advanced Inhale-Hold-Exhale Deep Breathing.

What is it Good For: Advanced deep breathing combines both of the benefits of the above two breathing exercises, making it great for those suffering from severe anxiety and panic attacks. However, it can be hard to master in a time of panic, so many people struggle to perform this type of exercise at first. If you're confident you can get the strength to use this exercise, you may find yourself able to calm down much faster.

How it Works: You'll need to find a much more comfortable place for this to work, and expect it to take a considerable amount of time. Find a quiet place you can stay for at least 15 to 20 minutes. Sit like you would for deep breathing with your back straight, but try to be comfortable.

For this exercise, you will be monitoring your heartbeat to keep a good rhythm. You will do at least 10 breath cycles, and each cycle will be comprised of three stages:

1. Inhale, count 5 heartbeats
2. Hold breath, count 7 heartbeats
3. Exhale, count 9 heartbeats

When you breathe in, make sure that you're breathing in through your stomach first and your chest second. Don't perform this exercise if you have a heart condition.

The slow, managed breaths are very calming. Holding your breath also helps regain your Co2 levels, to reduce some of the effects of hyperventilation.

Additional Thoughts: It can be very hard to perform this type of exercise, especially if it's your first time with breathing exercises. During an anxiety attack, it's hard to gather your thoughts enough to count heartbeats and calm your body. But if you can master this technique, you'll find that you should be able to calm yourself during a panic attack, and possibly experience some relief from your panic symptoms.