Another approach is to place a big beefy diode (1N4001 or bigger?) backwards across the supply rails - this will
conduct hard if the power is applied in reverse, and then the power supply itself has to deal with the problem
(current limited supply? fuse?).
The advantage of this protection circuit is that it doesn't have any impact on correct behaviour.
The disadvantage is that it assumes the power supply will back off gracefully (for instance a lead-acid battery won't).
Best to include a fuse too.