Meditation has worked well for my anxiety. Just lie in bed listening to relaxing music (noise canceling headphones work well for this) counting to 500 or even 1000. Everytime a negative thought comes into my head I just refocus on counting. It really helps in the moment to calm you down and over time, too. Seriously, this is probably the most effective way to calm down.
For times you are not having panic attacks: Journaling and intense exercise like running or cycling also helps get the negatives thoughts out of your head.