Instructing anyone who needs calming, autistic or otherwise, to count 1, 2, 3 and take a deep breath is an old well known technique. I mention it because it has been a particularly effective technique with Conor who is severely autistic and can get pretty wound up at times.
Conor loves his numbers, likes to do addition and "count by's" eg. count by 10,000"s, count by 3's, count by 7's etc. I have found that stretching out the 1,2, 3, breathe with 2,4,6, breathe, 3,6,9 breathe up to 10,20,30 breathe keeps Conor's interest in the activity and makes the 1,2,3 breathe calming technique even more effective. An old trick but one that is often very helpful.