If you are travelling by plane across multiple time zones you will inevitably feel some degree of jet lag upon your arrival.
Your body will be out of sync with the daylight and nighttime patterns of your destination.
It can take up to a couple of days for your internal clock to reset itself.
During this time you may feel fatigue or experience temporary insomnia.
More unpleasant symptoms may be irritability, difficulty concentrating, confusion, disorientation, and even constipation or diarrhea (as your bathroom schedule is also disrupted).
They are a handful of ways before your trip, during your flight, and after your arrival to speed up the reset process and in some cases drastically decrease the effects of jetlag.
Food and Drink

• Stay hydrated.
  The humidity on planes is very low and contributes to dehydration.
  Drink plenty of water, juice, or other non-alcoholic and caffeine-free beverages before and during your flight.
  
• Avoid alcoholic beverages.
  They will prevent quality sleep and will contribute to dehydration.
  
• If you are trying to stay awake on the flight, eat high-protein foods.
  If you are trying to sleep, load up on carbohydrates.
  

Sleep

• Establish a regular sleep pattern before you leave for your trip.
  If your biological clock is already out of sync, the effects of jet lag will be that much more profound.
  
• Begin shifting you sleep/wake cycle to the new time zone a few days before you leave.
  This may not be possible for longer trips, but can be quite easy for time differences of only a few hours.
  
• If possible, follow the sleep/wake cycle of your destination during your flight.
  If it is currently night time where you will be heading, get some sleep - especially if you have a full day ahead of you.
  

Exercise

• Unless your plan is to sleep, get up and move around every 1 - 2 hours during the flight.
  Stretch often to remove muscle stiffness and increase circulation.
  
• Exercise moderately as soon as possible after you arrive.
  This will reduce the stress of flying.
  If you are arriving late at night, exercise first thing the next day.
  

Other Tips

• Set your watch to the new time zone as soon as you depart.
  This will get your thinking in sync with the time zone of your destination.
  
• Make an effort to enjoy the flight.
  Do not let minor inconveniences bother you.
  This will only add to the stress of the flight making for a longer jet lag recovery.