Jetlag is an almost unavoidable pain when flying more than 8 hours, and having just returned from California (to Europe) I thought I'd share my tips for mitigating the worst of the effect.
If you can, start a day or two before the flight by adjust your bed-time an hour or so towards the destination time zone.
Engage the fasting clock
Scientists have determined from studies in mice that the lack of food for 12 - 16 hours can trigger a reset of your second body clock (in the dorsomedial nucleus). The theory is that if you're hungry then getting access to food becomes more important than getting sleep while it is dark.
Don't eat for twelve to sixteen hours before breakfast time in the new time zone—at which point you have breakfast (or the next normal meal) at the correct time for your destination. The second most beneficial side effect of this technique is you can avoid the less than fantastic airline food (I'm talking about you United!)
I usually eat a light meal before getting on the flight, and try to drink no caffeine and little alcohol as both disrupt sleep and rest hormones (again - not a problem on United as they charge for beer and wine).
Drink plenty of water before, during and after the flight to avoid dehydration.
Clothing and an eye-mask
Make sure you've got comfortable clothing ideally something with pockets on the thighs to give you storage options other than the back pocket. Take off your shoes and add a pair of over-socks for comfort and warmth.
Personally I don't use neck cushions as I never found one that was comfortable, however noise-cancelling, noise-isolating earphones or a good pair of foam ear plugs are essential for sleep.
When travelling during the night in your destination time-zone you'll ideally want to avoid light from the cabin or seat-back screens (to help re-set your circadian clock - a tiny cluster of light sensitive cells in the brain called the suprachiasmatic nucleus into the right time-zone) so use an eye-mask This has the additional benefit of signalling to most cabin crew that you'd prefer to sleep than eat.
Sleep is key
Most recommend not using artificial sleep aids, however I find that once airborne (so there's little risk of having to return to the gate) a couple of Advil PM tablets knock out some of the back-pain caused by sleeping in an economy seat, and help me to drift off. Best used only on a long (>10 hours) flight, and only if you're not planning to drive at the other end (which is always a bad idea anyway). Advil PM can also be useful for the first night or two to ensure you get the full 7.5 hours (although I avoid it if possible, as I find that I get better quality sleep without it).
I know some folks rave about taking supplemental Melatonin (the body's go to sleep chemical signal) however I've never tried it myself.
If you do wake up on the flight try to go back to sleep. Stay asleep until about an hour or two before landing and then drink plenty of water.
On a westbound flight, I usually try to take a day-time flight and catch a few hours sleep during the flight but I don't use sleep-aids. When I arrive in the afternoon/evening, I always try to eat a normal meal at the correct time, and stay up until at least 9pm or 10pm.
If you're going east back to the UK, try to return at around midday to land at the brightest light of the day. I also like to make sure my flight out of the US is as late as possible, so that I'm ready to sleep, but this isn't always easy.
On arrival - exercise and a power-nap can help
A short episode of moderately intense exercise may help reset your body clock after arrival. Don't go to sleep before bedtime, but a power nap for no more than 30 minutes (set an alarm to wake up), followed by coffee may help to recover alertness, and make you more productive until it is time top hit the sack.