Man, just say no. Make friends with the FR, let them know that maybe you want to be on the bottom of the list or, if you're like me, to not call at all. I've answered a few times when I was at the bar and those are hilarious conversations, especially when they say they wish they could join me. If your schedule is really important, you can also set up a relationship where you show initiative and call in requesting shifts or just asking ahead if that kind of flexibility is available (which it is, sometimes, but don't rely on this process too much).
I do have my moments of weakness though, my favorite SL TL calling me = yes, but she also knows to never abuse that privilege so I know if she's calling me she's really in a bind. My ETL/STL? That is a no, always.
