In a speech at the Resource Alliance's International Fundraising Congress in the Netherlands, Jennie Thompson urged charities "to keep new supporters giving after the first crucial gift".
She argued that it is much cheaper to keep a donor active than it is to reactivate a lapsed supporter.
Charities often pour huge amounts of money into attracting new supporters, only to find that a large percentage never give again. A common reason cited by donors for ending the relationship is that they feel they are not being listened to by the charity.
Thompson advised charities to make more of an effort to get to know their donors and find out what they are looking for in terms of feedback. She said: "The donor wants to feel as if they are on the inside of a charity, and that they are being given valuable information from the organisation.
"They don't want charts and finances - they want to know what the charity has achieved in meeting a specific need."
At the root of the problem, said Thompson, is the inability of many charities to say thank you properly. She cited a recent survey in which 61 per cent of donors said they had received a request for a second gift with an accompanying note of thanks. Of those, 81 per cent were offended by the request.
Thompson said the thank you should be just that, not part of a request for further donations. It should also, she said, be more immediate and include reassurance that the gift will be used for a specific purpose.
"The next time you ask for a gift, you need to be able to show the donor how their previous gift was used and how it has made a difference," she said.
Thompson referred to a school in New York to which she had made a donation.
She received personal and hand-written thank-you letters from the children, with photos of the school trip her money had helped fund.
"Saying 'thank you' is not just good manners; it's the first step in building a relationship," said Thompson.
"If you want to show off, be very good at thank yous. Not that many charities or organisations are."