Actually, the best part about the "you scratch my back and I'll scratch yours" is that it refers to whipping. It's an expression that comes from ship life, where shipmates would be tasked with whipping each other when they misbehaved. The idea was that if you whip me more softly (scratch my back), then when it's my turn to whip you I'll do the same.

http://eifuku.co.uk/etymology.htm#You%20scratch%20my%20back%...