Like Patrick, I started collecting MSS in India in the 1970s (but only 1978, so you beat me there, Patrick :-).  I echo everything that has been said by Adheesh and others about tact, patience, and etiquette.  These, more than money, more than authority, are key elements in successful MS work. 

If you are able to take digital photos, always take a CD burner with you and give the librarian a copy of the photos you have taken.  If you publish a book or article using the MS, remember to send a copy to the library. 

Where I think I would differ a little from Patrick and Adheesh is in saying that there was an early golden age that has turned sour in the 2000s.  Even back in the day, there were intransigent libraries and librarians.  I remember in about 1980, a close colleague sitting in the library at Thanjavur, day after day for over two weeks, until the librarian asked, "when are you leaving?"  My colleague answered, "when I have read the manuscript."  It was produced the next day.

A counter-example.  The largest MS library in the world is the Gyan Tirth at Koba, just on the outskirts of Ahmedabad. Yes, I mean it.  250k MSS, making it four times larger than the Vatican library or the BN in Paris.  I was there in late 2011.  The faculty and staff could not have been kinder or more helpful.  Everything computerized and efficient.  I was given PDFs on my data plug within half an hour of asking.  No money.  And I was told, "next time, no need to come so far; just send email, we'll send PDF as attachment."   Utterly amazing. 

So, it goes both ways.  Government institutions are commonly problematic: slow, rule-bound, often expensive (Baroda!).  Private institutions can go both ways - some are impossible (Bikaner) others are fabulous (Koba).  As the recent filming in J&K by Chetan Pandey demonstrates, local knowledge and language skills can be hugely enabling.