What's the best way to calm racing thoughts?

Stay active. Exercise is a great way to relieve stress. Your body releases endorphins during exercise, which can help you feel calm. If you exercise 30 - 60 minutes a day, your stress levels can improve. When stressed, take 5 slow, deep breaths with your eyes closed, then roll your shoulders forward 5 times, then back 5 times. This will slow your heart rate and release tension in your neck and shoulders.
Medication. Racing thoughts can occur in several disorders. I would recommend seeing a psychiatrist to determine the correct diagnosis and then the best treatment course.