I did longhaul for 24 years. You can't practically, with a family, 'train' yourself in the days prior. Sometimes you may be lucky to sleep late or grab a nap before leaving. The long long flights have a 1/3 rest time. I usually used to find I could go long into the flight before crumpling, often a short period of extreme fatigue could be shrugged off and pass. Some people try and stay on UK time, but I always hated sleeping on sunny days, so I always slipped into local time with judicious naps.