I've traveled on long business trips between the US and India and picked up some tips that may help.

Tips for the flight:

http://www.flickr.com/photos/ben-zvan-photography/215568199/
Hydration:
Buy a bottle of water at a concession stand near the gate so that you can take it with you on the flight. Every time you are offered a drink by an attendant, get a cup of water in addition to whatever you really want, and avoid anything that will dehydrate you (coffee, alcohol, etc.) In addition to drinking what is offered, drink your bottle of water too. The recycled air will suck the moisture out of you, and if you get dehydrated it will make you feel tired, increase your vulnerability to illness and increase the chance of a jet lag headache.

http://www.flickr.com/photos/vic06/3731783559/
Hearing Protection:
The high frequency noise of the plane moving through the air and the engine noise have been scientifically proven to increase fatigue and symptoms of jet lag. I insert soft foam earplugs in my ears, and then use over the air can-style active noise canceling headphones on top of them. It blocks most of the noise, and I can plug the outer head phones into my iPod and enjoy an audio book while I try to nap. I use the headphones in the source link as they are fine for the above purpose and are a LOT cheaper than the popular Bose version. You could probably use quality isolating earbuds for a similar effect, but I prefer doubling up by using ear plugs and active noise cancellation together. Whatever you do, don't bring ordinary headphones and then blast the volume to drown out the plane or you'll experience some permanent hearing loss by the time you land.

http://www.goodhope.org.uk/Departments/eyedept/images/celuvisc.jpg
Eye Protection:
Bring disposable single use size eye drops with you on the plane. The thicker and more viscous the better. I like Refresh Celluvisc, Refresh Plus or Systane in order of preference. Celluvisc will feel almost like dropping oil into your eyes, but it gives them a nice lubricating coating that will help prevent them from being irritated by the dry air. I'd use them before you take off and twice more during the flight.

Body Maintenance:
Get up and use the restroom a few times during the flight, even if you don't absolutely need to. The act of standing up, stretching and walking around a bit will help keep you from stiffening up and is especially important if you are a large person stuck in coach. Doing this will help prevent soreness and keep you mobile your first morning after arrival. If you tend to get dry mouth, consider bringing a travel size bottle of Biotene Mouthwash and using it to freshen up.
http://www.biotene.com/products/productimages/mouthwashBackground.jpg

http://www.flickr.com/photos/l0ttie/2508193473/
Bring Lozenges:
I like to bring a pack of mild throat lozenges, such as Ricola herbal mint drops to stave of dryness. They also come in handy as you can push one onto your neighbor if they have bad breath.

Before You Land:
Fill out your customs and immigration paperwork while you have the convenient tray table in front of you and before you are stuck standing in line with nothing to write on. Gather your passport, baggage claim ticket and customs documents somewhere that is readily accessible.

When You Land:
You likely will feel tired, groggy and not very alert. Double check your seat and storage areas carefully to make sure you don't forget anything. If you need to call anyone, do so before you get to the customs area as they are pretty strict about not letting anyone use or handle their phones near the immigration and security area.

After clearing customs:
No matter how tired you feel, do not nap, sleep or lay down until the local bed time. A few hours of unhappy tiredness is well worth helping you quickly adjust to the local time. Find something active to do outside that lets you see the sun (or lack of) until the local bed time. Walk the streets and look at local shops or take a simple tour. Exposing your body to natural sunlight really helps re-set your internal clock.

1 hour before bed time:
Dim the lights slightly, put on some relaxing music or an audio book and crank up the AC in your room to lower the temperature. Don't look at any back lit screens, gadgets or TV as the light will make it harder to wind down for sleep. If you are WIDE awake and certain you won't be able to fall asleep later, consider taking a gentle over the counter sleeping pill or having a drink. Set a couple of alarms for when you want to wake up in the morning, and possibly schedule a wake up call for back up.

First Morning:
When the alarms go off, immediately force yourself out of bed, even before you take time to think about how tired you feel and before you are tempted to hit the snooze button. Open the curtains and flood the room with sunlight. Take a shower and get ready to take on the day. If you feel the need, have some coffee or an energy drink with breakfast. Don’t get hyper-caffeinated though or you'll just crash again later. As soon as possible, take a brief walk outside in the sun.

Lunch:
I usually start fading by my first afternoon when I jump a lot of time zones. Thus, a cup of tea or coffee with lunch might help, providing it's an early lunch and that you will have many hours for it to leave your system before bed time.

http://www.flickr.com/photos/wanhoff/226318202/
Evening:
Indulge in a relaxing hot bath, Jacuzzi tub or even a massage if you can afford it. You'll sleep like a baby.

If I make it through my first day with the above technique, I'm usually fine and the jet lag doesn't bother too much for the rest of the trip. I admit that for my first two nights I take a mild sleeping pill, and my first 2 mornings I take a 5hr energy drink to help me adjust, but that's because I usually need to be sharp for business meetings the morning after I land.
http://www.flickr.com/photos/paulmichaels79uf/2383173772/

Overcoming jet lag:

When possible, it helps to choose flight times that work with your current sleeping schedule, it will make the time change easier, for example:

If you can sleep on the plane, depart your city of origin at a time that coincides with night/evening in your destination city, you'll arrive refreshed in the morning.

If you cannot sleep on the plane, depart your city of origin at a time that coincides with day/morning in your destination city, you'll arrive in the evening.

Overall, the most effective way to adjust to the time change is to start keeping "normal hours" locally as soon as possible. Get physical activity during the day so you'll be worn out by the end of the day, and ready to sleep.

Avoid illness from airplane:

Take a small bottle of hand sanitizer (TSA will permit liquids totaling 3.4oz or less). Use it to wipe down surfaces you have to touch (buttons on seat, tray table, bathroom door, etc) and use it on your hands regularly.

Some people suggest taking "cold prevention" medicines like "Airborne" ... I'm not sure how much good this does.

Don't point the air vent directly at your head (you don't want cold air blowing directly on you for hours).

Don't drink the water/ice on the airplane. Some studies have shown that ice/water on airplanes is less than stellar quality. You definitely need to stay hydrated, consider purchasing a water from a kiosk (post-security, so they don't confiscate it), or drink juice/soda sans ice on the plane.

I also recommend taking lotion and chapstick, because the air on the plane really drys out your skin.

Immigration and security:

For immigration, keep all of your documents (passport, landing cards, drivers license) in a safe place that is easily accessible to you, this should speed up the process when it's your turn to go through immigration. You do not need a visa to enter the UK from the US if you are staying less than 6 months.

Before you land in the UK, airline staff will provide a "landing card" for you to fill out. It will ask for information including: flight number, name, birth date, gender, nationality, occupation, full address while staying in UK. When you go through UK customs, they will enter the information you provided on the card, and stamp your passport with an entry stamp and date.

On your return trip airline staff will pass out customs forms for you to fill out. On the form you need to declare what you are bringing back into the country (for tax purposes). When re-entering the US, you can bring up to $800 worth of goods without being taxed. For more information on what you can bring back, see the US Customs website.

Traveling light is best. If you cannot travel light, try to have your carry-on belongings as organized as possible (remove laptop from bag, remove shoes, remove loose change, keys, etc) when it's your turn to go through line.

When you depart the US for the UK, the process should not be much different from a domestic flight. On your return from the UK, your bags may or may not be selected for search when you go through customs.

I hope you have a great trip!

what country ?

first of all when you are going to travel by plane is to be CALM, you want to have an easy, enjoyable journey, bring important stuff you need like medication, specific clothing, whatever you think are necessary. just follow the rules when boarding, custom, immigration procedure. make sure you don't have any illegal stuff in your luggage, pack your own thing and lock them well.

when you have arrive at your destination, again you have to follow the laws of that country to avoid problems that ruin your trip. For your safety avoid jay walking for it is the most common offense by traveler. as the roman says: when in rome , do as the romans do.

Try to pack light. Lugging baggage through international terminals can be hard.
Immigration is smooth if you have ALL of your documents for each person in separate plastic bags for each person in your group. To keep from getting sick from the air, try to eat as little salt as possible; it will dehydrate you. Alcohol and caffeine also can dehydrate you. If you are flying within the US, wear socks; you have to take off your shoes when going through security. Get a seat near the front of your cabin.

To avoid getting sick bring a lot of thin shirts, it's important never to be cold or to hot. Therefor you need enough clothes to regulate your body heat at all times during the flight.

When going to UK also remember that they drive in the opposite dirrection that you are used to. Fortunately they have signs at all the pathways which makes you remember to look in the right direction before crossing the road.

To adjust to the time change try to change it before leaving home - if possible