millimummy - I am not against women using herbs or homeopathy or even magic spells or keeping their fingers crossed to improve milk supply...all of which, as you say, have been used by women faced with this issue. I pointed out that the only good evidence of something working is more freq. effective removal of milk (plus domperidone used in conjunction with more freq. effective removal). Deliberatelt increased water intake does not affect supply.
Refs for stress having no effect on supply (yes, it can have a time-limited effect on letdown - but that's not what you said. In fact you said 'of course' it can affect supply):
J Hum Lact. 2006 Aug;22(3):305-14.
Mood states and milk output in lactating mothers of preterm and term infants
West J Nurs Res. 2005 Oct;27(6):676-93
Psychological distress and milk volume in lactating mothers.
There are other papers, too.
In addition, we know from knowledge and observation that stress does not generally reduce supply - the human race continues to undergo enormously stressful experiences. This is the norm for our species.
It makes no evolutionary sense for the nutrition of the most vulnerable members of the species to be dependent on a lack of stress in the supplier .
Nature happily over-rides stress in order to keep our milk production chugging over - though letdown as I say can be affected in the short term (this is thought to be because the biochemistry of acute shock and fear interferes with the biochemistry of letdown...prob as mammalian leftover, a protective measure in mammals, so if you were running away from a predator, you would not leave a trail of dripped milk. Speculation only).
Hope this helps. What definitely does not help is telling women not to be stressed or their supply will be affected - disheartening, and wrong