How would you feel if the other side was doing it?

I'm currently obsessed with how to design a political system that actually works. I haven't experienced one in real life, but that shouldn't stop us from considering what one would look like.

A few minutes ago, I was watching an interview with Michael Moore where he talks about how Republicans are really secure in their seats, and Democrats only have control in 6 out of the 50 state capitals.

In it, he also brings up gerrymandering... the redrawing of districts to almost guarantee that your candidate wins.

Gerrymandering is so obviously bad and shouldn't be allowed. So how can we design a system that makes this kind of thing not possible, without having to write rules for each specific thing that someone might come up with?

How about the rule of "how would you like it if we did it to you?"

Sort-of like when you're kids, and the rule is the kid who cuts the cake gets to choose her piece last. Or a shotgun clause in a contract.

If you wouldn't find it fair if the other side did it to you, then it shouldn't be allowed.

I'm not sure exactly how to design or word this, but it seems like a good starting point for a definition.