Too many charities are defensive and fail in their duty to be accountable to supporters, according to Brian Lamb, the RNID's director of communications.
The RNID has produced a guide based on its own best practice called The Value of Impact Reporting, which it is making available free to other charities.
The RNID has been producing impact reports since 2006 and has won a number of awards for its work in this area. It decided to share its experience after receiving requests for advice from other organisations.
"What we are saying is really quite obvious," said Lamb. "A lot of charities think they are being transparent, but their annual reports are eulogistic, with pictures of smiling people and no real analysis.
"The evaluation business has become very sophisticated, but there are lots of things even small charities can do to be more open."
Lamb claimed that charities should not consider impact reporting to be optional and that they had a "moral obligation" to their supporters to be more transparent and accountable.
"The public often trusts us without understanding what we do, but we have a duty to show how we are using their time and money," he said. "It's not enough to say how much you spent; you need to show return on investment."
For a copy of The Value of Impact Reporting, call 0808 808 0123.