As this cause is something I feel passionately about, I try to find little ways to send money their way - I recycled my old mobile phone in one of their envelopes, so that they got a donation, and I buy their beautiful tree calendar each year.
And I thought that maybe by sharing the link, I could point other like-minded folk in their direction... the website explains lots of things you can do to support The Woodland Trust:
http://www.woodland-trust.org.uk/

and is a fantastic resource for everything concerning trees!