As others have said, try and adjust your sleep patterns in the days beforehand (if possible). So if you need to stay awake longer on the flight, try going to bed later (even a few hours make a difference) and getting up later (work/other stuff permitting of course).

Avoid alcohol and caffeine and stay well hydrated.

Take melatonin/some other sleep aid if you're struggling for sleep. Talk to your doctor and if you have time, try a few different options and see which one works best for you (all sleeping aids affect people differently).

Having flown TPAC plenty of times, your problem won't be DFW-SYD, it will be the way back as you'll have to force yourself to stay awake.