How to give money to good causes

It is the season for giving. Nearly two-fifths of Americans tell pollsters that they sign over more of their savings to charity in the holiday season than at any other time of year; more than two-thirds of Britons say they plan to donate money at Christmas. But the labyrinth of charities, good causes and people in need can be as hard to navigate as the seasonal family get-together. How to give well?