Breath in for 4 seconds.
Hold for 7 seconds.
Breathe out for 8 seconds.

Repeat until you feel calmer.

My strategy is a bit long, but it has worked for me:

1. Write down a list of everything you are stressed about. This will allow you to vent
2. Take some time to breathe, read a book, or do something active (the third one especially works to get rid of stress energy)
3. Go back to your list and look at each thing. How many of these are real issues, and how many are blown up to be bigger than they seem? Remove any items from the list in the 2nd category.
4. Start with one item on the list and continue from there. Make sure to take breaks; doing work for so long leads to distraction.