(as learned via the harshest teacher, M. Experience)
Don't forget your photo ID at home. (Forgotten before a flight to Montreal, 2009. Remembered upon reaching the [probably don’t need to say departure, but doesn’t quite read as redundant either] airport. Lucked out as I had time to go back and retrieve it.)
Keep your pockets empty, don't wear any metal (hair clips included).
Coins tend to be more weight than worth.
A heavy scarf is almost always worth it.
How many home keys do you need with you?
Snag a couple of fresh bags for liquids and sort out the liquids, gels, and aerosols before security (ideally at home) in one of those. Save the other for next time.
Security routine: three trays or more (maybe fewer if you’re at Rome Fiumicino FCO in 2025; they may not care for taking laptops out of sleeves — or backpacks, for that matter). If you’re at Bucharest Otopeni OTP, please be advised that there is no such thing as a system or line-up or anything of the sort when it comes to trays.
outer layer: coat, scarf, etc. + liquids
electronics (maybe drop your ID & boarding pass in one of these trays too)
the “personal item”
the carry-on bag
If you have “no known allergies” and you’re considering taking Benadryl (or the generic diphenhydramine) to sleep, maybe back that up with a different class of OTC anti-allergic. (Broke out in hives after Benadryl, 2019. To this day suspecting an allergic response to diphenhydramine.)
Don’t count on the city’s taxicabs to be willing to pop by the train station to pick you up, regardless of whether you may be going downtown or not. (Made the error of assuming that taxicabs serve the entire city in Charleston, SC, Anno Domini 2023.)
If you’re travelling with anything that needs refrigeration (e.g., medication), and have a long layover, maybe try to find out beforehand where/if the airport may have a first-aid/medical station. They might be willing to keep that in their fridge for a few hours. (Some airports do. 2022.)
Don't lose your credit card on the first day of travel. (Done that in Queens, NY, 2024.)
That’s all I can remember for now, but this is why I only published these on web.