1. Do Your Research.
With the presence of the internet, there is no excuse for not being prepared. Know the documentation (ie, passports, I.D., medical notes for drugs, needles etc.) required in the area you are going to. Find out whether any special vaccinations are necessary. The CDC is my go to site for this sort of information.(CDC Travellers Site).
Additionally, don't show up without learning a bit about the culture, climate, places to see and to avoid. Sites like Tripadvisor can be useful, but always take the info with a grain of salt. (Nothing is quite as good or bad as people like to make it out to be!)
2. Pack Smartly.
Now that you know what the weather will be like, lay out your stuff. Stick to neutral, easy matching, multi-purpose clothing, if possible. Lay out everything you think you need - then put half of it back. . . Do you really need 12 pair of shoes for a weekend trip?
If you are flying, give serious consideration as to whether you are able to pack what you need into a carry-on only. Not only will you be able to get out of the airport faster, but you will likely save some money by avoiding checking any bags!
3. Adjust to Time Changes ASAP
If you are crossing multiple time zones, especially moving from west to east, it can take up to a full day per hour change to re-adjust. Once on the plane, switch your watch to the destination time zone, and try to follow the local time routine as soon as possible. Try to go to bed at the local time, even if you are not particularly tired, and try to eat at the local meal times as soon as you can. There is some evidence melatonin may be of some benefit with time changes, as well as light exposure during the appropriate destination times, but the studies are not strong enough, so I will not make reference to them. No harm in trying though, especially if you know you have difficulty adjusting.
4. Hydrate, Hydrate, Hydrate
Sitting in a vehicle, be it a plane, train or automobile, often takes us out of our usual hydration routine. Additionally, planes can be quite dry environments. Drink lots of water, and avoid alcohol.
5. Slow Down and Enjoy the Journey.
If you've prepared well, then, for God's sake, slow down. You're on vacation.
Have a great time!
What other tips do you have for travel?