Book on a flight which is completely full and requires a full spare crew to be sent as "must fly" to retrieve passengers stuck overseas.

Seriously. Anyone who tells you that they have an even remote clue is trying to outsmart RM/IM software for which AA paid $ hundreds of thousands. The flight that you book late on a Thursday xPHL because it as capacity now, may turn out to have 75 misconnecting passengers due to a delayed inbound from LHR.