Everyone will have a different take on how to tackle the 15 hour long haul section of this trip and the lengthy journey generally.  I’ve made 26 flights of this length over the past 18 years so while I’m not saying this is going to be one-size fits all, or that you’ll want to include as much booze (!) this is my tried-and-try approach (using the ‘flying from America’ example, as most of you are):

• After checking in, and divesting yourself of the big suitcase, have a Bloody Mary at the bar of which ever airport you first get to.  This is very important as it sets the tone for the rest of the trip.  Substitutes can really be any type of alcoholic bevy to get into the holiday mode, but I personally think a “breakfast” drink is best (mimosas are another good one) as it’s morning in Australia when you’re boarding in the States.
• Setting an alarm on your phone for when your next flight is boarding is a good idea too, stops the constant checking of time and allows you to relax even more.
• While sipping on this drink and setting the “don’t get too relaxed and miss a flight” alarm, change any time keeping devices to Australian time.  Yep, do this immediately, while on the first  leg of your travels: it helps your brain start getting ready, plus you can say “…ohhh, I’m drinking a bloody mary at 8am!” to further advance your brains reorientation.  Having a relaxing bevy at this point also helps you feel a bit drowsy on any internal connecting flights, which is good, because taking a wee nap on this first leg will help with this next piece of advice.
• When you depart from LAX, it’s normally between 9pm and midnight.  That means it’s completely the middle of the day Down Under.  This is my #1 hot tip: DO NOT go to sleep for as long as you can on the long haul.  I aim to stay awake for AT LEAST the first 8 hours of the flight.  I will watch 2-3 blockbuster movies (usually action or something really exciting and “awakening” for the first two, something a bit more sedate for the last one) or pick a TV series that I know is going to be really gripping for a marathon session.  I don’t eat all the food they give me (a less full belly means less tired) however I happily quaff a few free beers and couple of glasses of red wine (yes, I know this is contrary to advice from most travel sites, and perhaps if you just drank water you’d feel even perkier, but this is me we’re talking about!).
• Speaking of drinking: yes, you should drink at least a good few pints of water from the water bottle you cleverly bought with you and have nice and full so you don’t need to buzz the attendant or keep getting up for drinks.  However, make the drinking part of the flight (booze or H2O) occur in the first 3-4 hours then slow WAY down.  Then, when you are seriously droopy eyed and can’t hang on any longer, go and do a wee (yes, I am actually giving you this level of info) then whack an Ambien or some type of other proper, prescription sleep aid down your throat.  I don’t EVER take sleeping pills in normal life, but I swear to god, they have been the game changer for me on long hauls.  I did the first 10-12 long haul flights in my life without drugs and since then would not even consider it.  I equate it with “you could have dental surgery or give birth with no pain relief at all, but why the hell would you?”  To deny yourself the advances made by modern medicine while indulging in modern flight is a masochistic decision in my opinion (plus an opportunity for some lovely alliteration).  If you are really, truly opposed to (or unable to safely take) prescription medication, get the very best herbal / melatonin-y loaded natural sleep aid you can find.  Between being really knackered, the few glasses of vino, the drugs, and an empty bladder, you should get at least 6+ hours of solid sleep which is going to mean you wake up in time for brekkie, with only a final few hours to fly and the sun will be rising (further brain trickery as your body says, “…wow, that was all a bit weird but it seems to be morning!”)
• When you get to Australia, DO NOT TAKE A NAP DURING THE FIRST DAY.  This is the hardest thing because by mid-to-late afternoon you’re REALLY going to want too, but if you make this mistake in your first 24 hours, honestly, you will lose the next few days to a shitty sleep pattern that looks like this: “wow, I’m so freakin’ sleepy at 7pm I’ve actually just landed my face into my dinner in a restaurant” followed by “wow, I’ve just woken up feeling bright as a button and ready to have adventures but it’s only 4am!”.  On a holiday that might only be for a week, the combo of ‘really early to bed, disgustingly early to rise’ just sucks.  So – as soon as you arrive and get to your first destination, jump in the shower and wash the grime of transit off you.  Then go outside.  Keep moving.  Get as much sun on your face as humanly possible (while moving around – no napping!).  Go for a swim or a run or a spot of beach cricket.  Drink coffee, Red Bull or whatever is going to give you a push for another few hours or even consider taking a very small amount of prescription stimulant like Adarol (yes, I know I’m sounding like a pusher-man but if your doctor can give you a script for a couple of tablets for this specific use they will fundamentally change your experience).  Try not to drink too much booze on the first night (finally the voice of reason!).  Obviously a few coldies to wash the dust from your throat will be mandatory but waking up not hungover on day two is the other critical part to my preferred strategy.  I also keep another half an Ambien for the first and second nights of sleep.  If – even with all the other steps followed – I wake up before 5am on the first night, I take the little half a pill to knock me out for the next few hours.  Better to wake up at 9-to-10am on the first day after a slightly medicated snooze than the alternative, I promise!  By day two, you should have broken the back of the lag.  You’ll probably still wake early-ish (like 6am) but not stupidly early, allowing for a nice full day of fun and a close-to-normal bedtime after 10pm.  And that’s it!

This guy also has really good advice, I agree with all of the tips wholeheartedly except #4, but lets face it, I have been genetically engineered to drink booze so maybe following all of his tips would be a better idea!  As far as coming back, I honestly find it’s easier returning as far as feeling crappy.  Maybe it’s being sustained by happy memories of what always is an amazing trip 🙂