I recall being held at LAX for an hour at the start of a LAX-SYD flight, due to unusually light headwinds that would have caused us to break the curfew at SYD. Not a fun start to a 14+ hour flight. Coming back a couple of weeks later, we had a great tailwind and made the crossing in 12 hours. On that route, ATC have something called Flextrack that allows the pilots some latitude (literally!) to sniff around for the best winds en route and change course accordingly.