A good way to release tension built-up from stressful situations is to divert your attention and focus on an external non-stressful stimulus.
Next time you experience an anxiety attack, try to:
Close your eyes and draw an idyllic image; you could revisit a lovely memory or think of a place you wish to visit. Try to attain to all of its details and characteristics.
If closing your eyes is not possible, focus your attention on an object in your visual field and attain to its details (shape, color, material, etc.). If possible, describe what you see to someone else in as much detail as you can.
Try to distract yourself by engaging into an activity that requires mental effort. For example, solving a mathematical problem or counting backwards by 7.