I make this run (XXX-ATL/DTW-LAX-SYD-XXX & return) about 10 times a year, about every 4-6 weeks. I've made it in Econ, Econ Comfort, Premium Econ, & Business (both DL & VA); all I can really say is, 'you get used to it.'
I find the keys for me are relaxing in the LAX SC beforehand (as well as an In-N-Out run if there is time). And then knocking back a few in the SYD KIWI lounge beforehand. The Hot Dog cart ROCKS. ^
I don't mind the flights anymore; there are always bumps around the Equator, but when breakfast is served, you can know you're almost there soon. Sleeping is key both ways, as is showering upon arrival in both cities before connecting onward across these two similarly-sized continents.