#1 rule: don't be attracted to each other.
To the female TFPer's list: #4 I'd disagree with. The whole thing about FWBs is they're there to get your rocks off simultaneously because you both want/need a bootycall. If someone, on the other hand, rocks your world, then FWB gets knocked down to the B list. Exclusivity shouldn't have much to do with FWB's at all.