Modern automatic gearboxes are far more sophisticated than their predecessors, so when descending a hill you should find that if you control the vehicle speed using the brakes alone, the gearbox shifts down automatically as it recognises that the vehicle is travelling downhill.

Brake fade is also much less of an issue on modern cars, and although in the past the correct method to descend a hill was to select a lower gear, the vast majority of modern cars will be able to complete such a manoeuvre without experiencing fade. If you are concerned about this however you can brake slightly harder for short periods and release the brakes momentarily, giving an opportunity for the brakes to cool. It is also worth considering that brake pads and discs are far cheaper to replace than automatic gearboxes, so you may wish to put the load through the former rather than the latter.