I have done a few long haul flights and am off to Oz soon - I learnt the hard way not to drink alcohol as it swells up your feet so drink lots of water, wear loose clothing - specially loose socks, and get up as often as you can and swing your legs back and forth, wiggle your toes, do ankle rotations, and do as the other posts suggest too. If you have a stopover, makesure you keep walking around and you may even have time for a shower and/or a neck and shoulder massage too - I'm going to try that this time! Have a good trip.