In the Philanthropy UK newsletter, he writes: "When you've decided where you can add value, recognise that many people are probably doing good work already in the same area, so don't think you'll be the one who'll provide the Eureka moment."
Blair has set up four charities since resigning as Prime Minister: a faith foundation, a sports foundation in the north east and organisations addressing African governance and climate change.
He said charities should seek partnerships, and contrasted their cooperative ethos with politics. He advised them to keep goals simple and be ready to change them when they were achieved.
"Few things are more important to a just society than a strong and feisty charitable sector," he said. "Without the efforts of charities, faith groups, social entrepreneurs and voluntary bodies, British life would be poorer, less cohesive and less free."