One good technique i learnt during a childcare course of mine to use when you are stressed, your mind is racing or just need to calm down for some reason is:

Close your eyes...
* Breathe in and count to 5 in your head
* Hold the breath and count to 5 in your head
* Breathe out and count to 5 in your head
* Hold it and count to 5 in your head
Then repeat the above. Just keep repeating, not thinking about anything else except breathing and counting. You should find a noticeable difference in your heartrate and feel alot calmer after about 5 or so times of doing this.

Hope this helps.