Travel

Planning your trip

United Basic Economy (no carry-on option) and Spirit don't have flight flexibility. You can buy flight flexibility from Spirit.
On airlines that aren't Spirit and Frontier, you can often change your flight to have more time in a place. It's often worthwhile, because it costs substantially less than flying back to the location again.

Are you going to use public transit to get to and from the airport? Have you checked if public transit is open at the departure and arrival times?
If you aren't getting to and out of the airport via transit, have you accounted for the cost in time and money of your other transportation method? Some cities have an airport close to the city and an airport farther away, and the additional transportation cost of the farther way airport sometimes eats up the difference.
ITA Matrix is a flight search engine that is like an "advanced mode" for Google Flights. Since it doesn't rely on caching, it sometimes has more up-to-date results. It also allows more sophisticated querying.

United Airlines offers 5% off flights for young adults between 18 and 23 who book via the portal until December 31st, 2025.
Amtrak offers 15% off train tickets for students who book via the portal between 17 and 24.
Student Universe allows college students to book cheaper flights. (Not tested)

Pointhound and Roame are travel search engines intended for credit card point use. (Not tested)

High-end flights (ex. direct flights across an ocean and a continent) tend to get most value per point.

Have you set an alarm with time to get to airport + 30 minutes for last minute packing before your desired arrival time for the departure?

Bring a water bottle: Fill after security. Cabin humidity is very low (10-20%), causing dehydration. Drink water regularly during the flight. Large (preferably 24oz+).
Have enough electrolytes: Eat sodium and potassium rich foods before your flight, on the plane, or during any layover if you have trouble retaining water.
Pack extra medication: Bring additional doses, and store it in multiple compartments, in case you lose it or extend your trip. Keep in your onboard baggage for easy access.

Long-haul flights

Set your timezone to your destination once you board the plane and start eating and drinking in sync with it

Wear comfortable clothes you want to be in for a long time

Japan

Bring as many debit cards as possible in order to obtain cash and pay for transportation cards.