What is fair vs. being taken advantage of?

This is something I've been struggling with a lot at work lately. We are shortstaffed (again. Oh wait, I meant "still".) and we've been spread thin again. The Powers That Be have asked me to work on pretty much every weekend this month- I work Monday-Friday. I have been giving them one day of every other weekend. I struggle with some guilt with regards to this, because we have some emergency staff that are there All. The. Time. and I don't think they should have to be the only ones to suffer. On the other hand, I think TPTB need to get it in gear. One, they need to plan a little further ahead for Sunday morning than oh, say, Friday evening. People would be more likely to help if it didn't mean getting plans interrupted at the last moment. Two, they want people in there all day, every day, and people would be more likely to help if it didn't seem like every single day was a so-called emergency.

This sucks, because I want to do my fair share so that my fellow employees aren't run ragged either; the way I see it, if we all suffer a little then nobody has to suffer a lot. However, if I worked every day they have wanted me to I would have worked something like fourteen days straight. Um, NO. I already worked myself sick and into a perpetual flare once, never again. I need to quit that freaking job while I still have some shred of health left. Unfortunately, I've been having this same conversation with myself for something like a year and a half. :P