1. When booking travel online, clear your search history or use an incognito tab: some sites will up charge if they recognize your device has stopped there before (cookies).
2. Use the camera on your smartphone to document things: your luggage, important documents, the dent on the rental car, the damage to the hotel room, the hotel room number or your parking spot in a garage, even. In short, anything you hope you don't lose or would need to prove you had or, in the case of rentals and lodging, didn't do. Additional physical copies of documents are good, too, but a quick snap of them should keep you covered.
3. Inform your bank and credit card companies that you are traveling before you go: the last thing you want on vacation is to be “protected” from using your own money. Placing a travel alert on your funds beforehand will save you a lot of time on the phone and at least eight varieties of headaches.
4. Take spare cash and a spare credit card. Keep these separate from your wallet and primary bag.
5. When traveling somewhere with a primary language that isn't yours, try to learn a few basic phrases: even clumsily spoken, the attempt will get you a long way towards whatever it is you may need help with (where are the restrooms, the bank; how can I? Please, Thank you, Do you speak English? Excuse me for disturbing you, but…). Plus, it just seems like good manners.
6. When visiting religious sites, particularly active ones, err towards conservatism in dress and manner. Whether it's a temple, cathedral, or mosque, a head covering and cultural awareness demonstrate respect for others' beliefs and gratitude for the welcome you've been given. If there's any chance the spot you are in is sacred to another, it's not a tourist attraction just because you are there.
7. Consider packing at least one extension cord and/or multi-use adapter for all devices instead of a gnarly tangle of miscellaneous charging cords. And don't forget that many TVs now have USB ports: this can help you out in a pinch. And, if you're traveling outside of the country, a plug adapter is something you don't want to forget.
8. Don't pack anything you can't afford or bear to lose.
9. Carry the most essential items in your carry-on bag: medications, tech, money, spare underwear.
10. Don't forget to pack a basic first aid kit: anti-diarrheal medication; antacids, aspirins, band-aids, neosporin, cold and allergy medications. Similarly, pack a roll of toilet paper, a pocket pack of Kleenex, or some baby wipes: depending where you're going, toilet paper may not be a given or as abundant as you would like. Hand sanitizer is also helpful.
11. A couple dryer sheets in your luggage will help freshen things up; they also can be used to remove static from hair and clothing.
12. Extra zipper storage bags or re-used plastic grocery bags are great to have with you: they take up very little space and can be used for dirty laundry (or, when leaving, to separate the smaller clean pile from the larger need-washed pile), wet swimsuits or shoes, or to separate items for packing. (These are also good for holding many small, similar items — on the off-chance your bag gets looked through by security, it will go faster and more easily for TSA to see a million lip balms, hairpins, and your tweezers in a transparent bag then to shake all your clothes out looking for the metal.)
13. Roll your clothes. This saves an immense amount of space. If you want to get fancy, you can then put the rolled-up clothes in zip-top bags and remove the air (which will wrinkle things but will really save space).
14. If you're traveling as a couple, family, or group, you may want to consider splitting each person's items between different bags: if you suitcase or bag is lost in transit, then at least you'll still have something that's yours while you wait for your bag's return.
15. Don't take anything you have never worn before on your trip: vacation is not the time to learn that your new shoes or clothing needs broken in or is uncomfortable or unsuitable for, you know, life.
16. When packing and dressing, think in terms of layers: many lightweight layers will prove versatile in the widest range of temperatures and will also expand your wardrobe choices. And, if possible, travel to/from your destination wearing your bulkiest or heaviest items: the sweater, the coat, the heaviest shoes.
17. Just take a scarf. Wear it on the plane or train or bus or whatever, but a scarf can be a sarong, a wrap, a curtain, a blanket, a towel, you get the idea. Just pack your scarf. Seriously. (See: 6 Ways to Turn a Scarf into a Shirt without Sewing; 30 Ways to Use a Sarong)
18. Leave yourself enough time: don't overschedule your trip or plan too many of your hours. Stressing out to get places leaves no room to enjoy the experience or to be flexible when the unexpected happens or a snag crops up.