Hell, I over prepare for even a small trip but here's a short list of things to keep in mind before you go.
Double Check Flights
Sounds like a no brainer, but I forget almost every single time and while it's only chapped my ass a couple of times...it was really really bad. There was a storm over the English channel last December and I was already over a week into being terribly ill in France when we got to the airport to discover, our flight was cancelled.
Cue 3 hours of standing in line (Han stood. I laid on the cold marble floor of Charles de Gaulle airport and tried not to cry) waiting to talk to an attendant to get a new flight. Our patience was rewarded and we got on a flight the next morning (the british ladies in front of us were very rude to the person behind the counter...and they got booked on a flight 3 days later, so keep that in mind while talking to airport staff.)
When I remember, I usually will go to the airline's website and check in 24 hours in advance. I also have apps like Delta's that lets you view flight info in real time and if yours is delayed or cancelled, you can check the app and re-book yourself to a new flight.
Available on iPhone, iPad, Android & Windows
Check Your Lodging
I've heard horror stories of people booking on Third Party websites, only to arrive and have the hotel have no record of their staying there.
After I book a flight or hotel, I ALWAYS save a screenshot of the booking information screen at the end even if it's being emailed to me. You can never guarantee that everything will work 100,000% so I put my mind at ease by using the "Windows Snipping Tool"
Also, even if it seems a slight bit (lot bit) neurotic, I call all of our lodgings a few days in advance to ensure that everything is all good. It saved me once at Disneyland because they had misplaced our reservation. They found it because I had saved that screenshot with all of the info on it. *Insert smug face here*
I'm really not. Like at all.
I do these things to prevent the high stress situations but guess what? I find myself in them all the time regardless of how well I have planned. When our flight got cancelled in Paris, it all worked out but the next morning they changed terminals at the last minute. CDG is a massive airport. We ended up sprinting through at least 3 terminals before running on the plane as they were closing the door for departure. Obviously, that sucked but if we hadn't just gone with it, we would've have been purchasing new flights home.
Don't Over Plan
I plan parts of my trips to the T, like flights and lodging. After that, I don't plan. I'll research and have a general idea of what's up but really, it's about coming to a new place and just going with the flow of it for me. We did that in London after a heavily planned trip to Ireland and it was a blast. We just walked around the city. I had more fun doing that than going to the Tower of London.