Per Peter:
Most charities include a self addressed pre-paid postage envelope. If
it's a charity we know we'll never support financially, we write a short
note asking them remove us from their mailing list to save them the
expense of wasted mailings and save a few trees in the process. We mail
it back in their envelope. We've had good luck with that strategy.

I'm going to start doing that - seems like it *has* to work because very
reply via pre-paid envelope costs them money.
--
Pete Cresswell