I wish I would have sat each day and stopped to breathe and take it all in instead of rushing to the next museum, train station, hotel, etc...
I learned to pray before I did anything as it calmed my nerves!! I was so nervous about my first train ride and prayed that I would sit next to a safe person! When I found my assigned seat, I was seated next to a nun!!
Pack what you know you will wear and wash it often (in the sink or laundry-mat). I take a backpack now and thoroughly go through my clothes and pick the minimum of what I know that I will wear and feel comfy in them!
Know where your lodging is located. While researching my stay in Salzburg, I wanted to be in the heart of old town. I found the "perfect" pension that was in "short distance" to Mirabel Gardens and Gerstreidergasse. I was hoping for a walking distance but it ended up being several bus connections away!
The drinking fountains in Rome are delicious!! No need to buy bottle after bottle of water. Just keep filling up the bottle that you already have! The water is clean and refreshing!
Sleep on the plane if you can! I have always battled jet-lag even though trying a different "solution" each time. This next time, it will work for me!
Prepare, prepare, prepare! There's nothing worse than not knowing the history of where you are going!
Enjoy the scones and clotted cream in the UK!!! Yum!
Get to know the locals. Go out on the limb and meet people of the culture and learn from them!
Learn a bit of the language and customs before going on your trip.
Don't stress out! Breathe!!! Enjoy!!! Take in every moment because the trip goes by pretty fast!
HAVE FUN!!!! :)