I setup a monthly donation to GreenPeace a couple years ago and they billed me longer than I anticipated -
I recently had to re-arrange my finances to cover myself more effectively and cancel the membership. Canceling only took 2 minutes and they were very kind, if not trying to keep me donating.
I explained that I was consolidating my monthly expenditures and away it went.
I thought that was pretty simple and straightforward, so I set a reminder to re-evaluate and donate again - and made a blog post to let people know it was no sweat!