Unix and Mac and Windows all have ways of scheduling recurring tasks (Linux: crontab, Mac: crontab, or I think iCal can do it, Windows... I dunno).
So if you're a little bit geeky, you could arrange for "bitcoin sendtoaddress 1mJMYFAVORITECHARITY9R4 amount" to get run once a month. (just run bitcoin with the -server flag).