Although the charity recruited more volunteers in 2006, people are giving up less of their time, which pushes costs up.
The trust currently has 47,000 volunteers contributing 2.5 million hours of work a year, but in 2003 it had 43,000 contributing 2.6 million hours.
Alex Youel, head of customer care for the trust, said: "It is a trend that is becoming increasingly common in the sector. It raises significant challenges because it is both more time-consuming and more expensive.
"We are currently looking at proposals such as family working holidays to encourage people to give up more of their time."