Yesterday, I exchanged a few e-mails with a professional photographer to confirm the licencing status of some of his work on Commons. I discovered someone willing to confirm the licence, but evidently quite disgruntled by his experience of Commons. Two lessons can be learned from what I read:

1) We are victims of a paradox which forces us to be especially annoying with the most precious of our occasional contributors.
A significant proportion of the high-quality photographs of celebrities uploaded on Commons are copyvios. This forces us to be especially strident with copyright issues towards well-meant photographers. Short of the most courteous civility, repeated requests amount to downright harassment, and may appear to question the word of the uploader.
I don't have a magic formula to break the paradox itself, but we should make efforts to sensibilise our users:
DO:
* be extremely polite
* apologise for bothering people with seemingly superfluous paperwork
* apologise for seemingly doubting their word
* offer to help and advise personally if the user needs anything Commons-related
* formulise the request in such a way that a simple "OK" from the user is sufficient. Open-ended questions are creepy ("what next, my credit card number ?") and bothering ("how many bleeding mails will I have to send before they are content with what I gave them ?").
DON'T:
* assume that the user knows all of our rules. We are there to guide them.
* assume that the user is aware of problems that we encounter as Commons administrators (typically, that most photographs that look like his are copyvios).

2) There is definitely a trend of professional photographers to request credits under the image in articles. This is what they are accustomed to.
I (and a few others) think that we should make efforts to sensibilise our users to this. We can definitely afford to credit people in articles. This is a small concession which costs us very little and can benefit us greatly.

See http://fr.wikipedia.org/w/index.php?title=Aide:Ins%C3%A9rer_une_image&curid=198477&diff=25280757&oldid=24980217
for instance.

Cheers !
  -- Rama