Assuming both cars have negative battery earthing (not that I assume anything, especially if there is the likes of a Morris Minor involved):
- Connect positive of one car to the positive of the other.
- Connect the negative of the car with the good battery to some convenient point on the engine block of the car with the flat battery.
So why connect to the engine block instead of the battery terminal? I figured out that this might give a lower resistance path on the "jump" circuit by eliminating the resistance in the connection from the battery negative to the earthing point and providing a more-direct path to the starter motor. But this resistance would be very low any way - far less than the resistance in the jump leads.
Any better suggestions, anybody?