1) Packing List--I have a master packing list on my computer. For each trip, I copy and paste it into a new document. Then I start deleting the things I know I'm not going to take or change items in which I found something more useful. I then decide if I want to make those "changes" on the master list as well. Packing is a continually changing and improving exercise. During and after the trip, go through the list and see if there is anything you want to change for the next one.
If you are not used to packing or have made major changes, don't wait until the last minute to pack. Do this at least a few days ahead of time to make sure there will be no problems. I use a combination of packing cubes, sacs, ziplocs and pouches. I then play Tetris to make it work. (I sometimes even do this on paper first. )
2) Use a paper calendar to plan your trip. I use the internet to do much of my planning and find it easier to have my planning calendar in front of me rather than going back and forth between pages. I make a custom one here: Create a Calendar
3) Sign up for alerts--flight notifications, weather, local transport (I use Citymapper). This way, you are less likely to be surprised and if there is a problem you can plan alternatives ahead of time.
4) It's rare that a trip will go exactly as planned. Stay calm and be prepared to make changes. Be flexible.
5) Don't get all of your travel information from one source. One person's don't bother is another's don't miss. A guidebook is just a collection of information. It is not a bible that must be followed to the letter. In a similar regard, take reviews with a grain of salt. If anyone can review, like on TripAdvisor, realize many may be fake. Use sites like Booking.com for reviews since only people who have booked that particular establishment on their website can review. (You don't have to book with them but can read their reviews.)
6) Get good, padded insoles for your shoes. Your feet will thank you.
7) Don't just make a list of the major sites in big cities and stick to that. Explore less touristy neighhborhoods and towns. Allow yourself to get out of your comfort zone. You might surprise yourself.
8) It's your vacation, your time, you money. Do what you want to do. Go where you want to go. Take what you want to take. It doesn't matter what anyone here or anywhere else thinks. (Except for perhaps the people you are traveling with.)
9) It's okay to wear shorts except in places it's not.
10) And as stated numerous times in this thread--ziploc bags are great.