Sleep (as much as possible)
Eat (decently)
Exercise (cardio/HIIT)
As stated, stay hydrated with water and avoid caffeine 4hr before sleeping prior to the overnight duty period. After waking prior to the duty period, caffeine works great...the stronger the better.
As a cargo dog, 3mg melatonin doesn't seem to move the needle for me and really never has. I take 5 and sometimes 10mg with zero ill effect. It isn't Ambien, it won't put you to sleep and in my experience won't keep you asleep, but it will help you fall asleep. That said, I don't use it when I'm not working overnight and don't have issues sleeping without it.
YMMV, caveat emptor, etc...everybody's individual physiology is different and so are people's techniques for day sleeping and WOCL working.