I would love to know exactly how many miles I have racked up flying round and round the world in my lifetime.
Sometimes I can't even believe how many miles I fly in one calendar year.

Regardless of whether you are making your first long haul flight, or if you are a veteran, there is most definitely a way to make it an easy, (relatively) comfortable experience. I can't be bothered with jet lag, and at least on my outbound trip I want to arrive refreshed and ready to go.
So here's what I do

1. Make a sleep kit
Whether you buy one ready made, or make your own, set yourself up to win with everything you need to sleep.
An eye mask to block out the light
A neck pillow (although these days airline seats have head rests that totally support your head, so I almost never bother dragging one around with me)

Earphones. Get noise cancelling if you can afford them, like the Bose QC15 . Look for something that is comfortable and that covers your ears (as opposed to earbuds). Anything that blocks out the hum of the engines and as much of the other ambient noise as possible. If you don't have noise cancelling headphones, plug in to one of the yoga/zen/rainforest channels that all the airlines seem to have - it will help block unwanted noise and put you to sleep.
A blanket. I've traveled with the same giant pashmina for more than 12 years now. Its soft, very very lightweight, but warm. It also takes up no space at all.
Airline blankets can be scratchy, and don't always keep you warm enough. Some airlines even charge extra for a blanket. I prefer to bring my own.

2. Check Bags

It takes forever for a plane to board, largely because so many people drag on too much cabin baggage. Pack a bag with enough to last you a couple of days in case your bags don't arrive with you. Be mindful of security rules on sizes of liquids etc. Packing only what you need to get you through the first couple of days in your cabin bag and checking the rest can streamline your experience getting through security and onboard the plane.
Rules change from airport to airport, so rather than deal with any hassles at the xray, I prefer to check as much as possible.

3. Drink Tons Of Water

Probably the most important item of all is to drink as much water as possible. The biggest part of jet-lag is getting dehydrated. You lose an additional 8 oz of water for every hour that you are up there, so its crucial that you drink as close to a glass of water for every hour of the flight.
4. Pack Healthy Snacks
Or at least check the airline's menu ahead of time. Sometimes there are healthy options - but mostly not.
Pre-sliced fresh fruit, unsalted nuts etc will serve you better than the high carb, high sugar, highly processed snacks and foods on most long haul flights. Be mindful of how much you eat too. Its super easy to overeat when you're sitting there for 10 or 12 hours.

5. Dress For Success
Wear comfortable clothes that you can layer up and layer down. Be able to work with however hot or cold the cabin is both during flight and also when sitting on the tarmac. Comfortable shoes (and socks!) are a must too. I'm a big believer in flying in comfortable clothes, but also in always looking stylish and well put together. 
Airlines frequently oversell seats and have to upgrade passengers. I always want that upgrade. No matter how much of a model citizen you are, if you're not dressed nicely you won't get that upgrade.

6. Sit Down, Shut Up, Powerdown. 
The flight can't take off until everyone is seated, belted in, and all electronics are powered down.
More so on domestic flights rather than internationals it seems to take forever for people to just sit down and do as they're told. And there is invariably some tool who feels the need to talk on his cellphone at full volume throughout the process. 
Make it easier on yourself and everyone else by streamlining the process. Once you're in your seat don't get back up to get anything out of the overhead bin until the flight has taken off and the seatbelt lights are turned off.