Check your reservation often online. They might change "equipment" (the plane slated for your flight) which will mean shifts in seats. The earlier you catch it the better your chances of staying together. This also shows you changes in time. If the change is significant (an hour-ish or more) you can get that changed to different flights for no fee (at least on the major airlines).
If you book with a small airline like Spirit with limited flights per day to your location, watch like a hawk and have backup plans. If your flight is delayed or cancelled you might not get out until the next day or worse.
Watch for weather EVERYWHERE. The tornado warnings the other week in NYC caused 3000 delayed flights and 700 cancelled flights. DH and I weren't watching for weather, even though he was in NYC. We should have been watching the airPORT websites and airLINE websites. One of the airports in the area had warnings up something like 3 hours before he even got to the airport (and he got there early). If he had known he would ahve realized that his flight might have problems.
Don't rely on a TA to change it when weather happens. GEt into the physical line while calling the TA. That way if the TA can't help (and his corporate TA couldn't, and didn't even think to route him to a different state with a shorter drive to his destination, and they focused solely on Atlanta b/c he was going to Georgia) you're still in line. Once the TA call was done he could have called the airline while still standing in line, so that he's in two lines at the same time.
Always have snacks or
budget for snacks available. Water, too. Make sure you have any medication you might need. NEVER put those into your checked luggage. If traveling internationally and you take odd medications, make sure they are allowed. DH was taking HCG for a few years to help mitigate the effects of a pituitary tumor; that's an odd one for men to take, and it's ONLY allowed in Australia (and Singapore) with a prescription. His bottles come in boxes with the Rx label on them, so that worked.
Put what you will need for a day or two into your carryon luggage, just in case.
If you're traveling for a cruise and there's no way to get you to the cruise if you miss that flight, KNOW THAT. Don't take the last flight out if there are no other flights that can get you there. If you're on a closed loop cruise and the islands the ship is going to cannot let you board there (Jones Act? PVSA? one of them is the right one to look at for passengers...the other is about freight, I believe), you might have to board very late in the cruise or you can't at all.
Just be watchful of everything. Don't rely on the airline to tell you anything; check check and check some more.