Exactly the technique I used to use target shooting to minimise the shake of the rifle

That is the key thing.
If you target shoot, that is the same exact technique you want to use here too. Good that cameras don't give you a kick back...

Gordon where did you get that concept of increasing pulse rate with the inspiration? I thought at the end of inspiration due to higher venous return to heart by increase is size of right ventricle (Expanded chest), you get higher ventricular ejection fraction and volume (End diastolic volume), therefore the pulse rate actually comes down to keep cardiac out put the same... Muscle tension is a different story though.

It's a long while since I studied physiology, but I'll do my best! There is an increase in venous return to the right side of the heart from the peripheral circulation caused by the lowering of intrathoracic presure on inspiration. Pulmonary circulation is not affected as it is entirely intrathoracic. I dont think there is a significant increase in volume of the heart chambers in inspiration (I could be wrong here, from a distance of 20yrs, but if I remember, I'll check next week when I can get at the books at work) The pulse rate increases towards mid-inspiration as a result of increased return (and thus helps get more blood through the pulmonary circulation) and then falls back at peak inspiration. On expiration, intrathoracic pressure rises, reducing peripheral venous return, and heart rate falls. If an expired breath is held for a long time, pulse will accelerate again in response to hyopxia.
I may well have the mechanisms wrong, but I do remember the effect as I was the victim for the demonstration! Just checking empirically now, and it still happens! (Though again from memory, the effect is more pronounced in young and fit cardiovascular systems; maybe there's hope for me yet as 40 closes in!)