1.) Make sure you pack your liquids in an easily accessible spot. If you're traveling with just a carry on, you'll have to pull these out before you go through security, and you'll thank yourself if you don't have to dig through your bag to get to them.
2.) Pack a healthy snack for the airport. You don't want to get stuck buying overpriced snacks from Hudson News just because you were too lazy to throw some almonds and a granola bar in your bag.
3.) Assume your flight will be delayed and make peace with it. This is especially useful if you're traveling to or from a high volume airport, as weather all over the country can have an impact on your flight, not to mention all the other things that go wrong just because those are the kind of things that happen to you.
4.) Leave half an hour before you think you need to. That way, you won't get too stressed out if your cab is late, the train doesn't come on time, the security line is extra long, etc.
5.) Always pack twice as much underwear as you think you need. No one ever regretted having too many pairs of underwear.
What are some of your travel musts?