The ethical dilemma du jour is if I should volunteer for a clinic shift this weekend, and if I should be volunteering more often anyway. I'm leaning towards "not" as it's going to be a long weekend and I've got a crapload to do, but... well, but.

It's just as possible to burn out on volunteer work as anything else. If you need to occasionally cut yourself slack so that you don't one day declare, "I hate you guys, I'm going home!" Than do cut yourself that slack. Remember you want to be a long term volunteer, not a long weekend one.