"It's all very well issuing documents from Whitehall, but that doesn't make it happen," he told a London conference in what was probably his final speech as third sector minister.
The second lesson, he said, was for innovative third sector organisations to be better connected to public service commissioners.
"Thirdly, there's a challenge for the sector in demonstrating impact," Miliband said. Good third sector practice should be fed back to the public sector, and the sector's right to campaign needed to be protected.
Miliband is tipped for a job in Gordon Brown's cabinet, which will be announced later this week. Emily Thornberry, MP for Islington South and Finsbury, James Purnell, a junior minister at the Department for Work and Pensions, and Jamie Reed, MP for Copeland, have been named as potential successors.