I'd just say that not getting hammered on the free booze, eating in moderation (not much choice on BA), and hydrating a lot are the keys.

I've done Singapore a lot in all classes. I can't quite speak for the full BA experience to Australia, except I did a W to Sin and then MH in Y to Melbourne. The biggest challenge I'd identify is the air quality for that length of time isn't very good, you get tired, your eyes dry up a bit, if you drink at the start you get a bit hungover at the end of a leg, or you feel bloated because of the food. So whatever your class of travel, you work on these issues and you'll feel better than if you didn't.