Pack your patience at the airports. Going through customs and security can take a very long time depending on when you are flying. Fly as early morning as possible to avoid the longgggggg lines.
Find out which airlines partner with each other. If we booked directly through Delta then they wanted like $2K per passenger to fly home from Rome. I was able to back door the same flight for $1K per passenger by using KLM's website.
If possible avoid flying Lufthansa unless they really get their act together.
Singapore Airline is a great airline to fly.
If you bring a CPAP machine then check in small bottles of distilled water in your checked baggage or order if from
Amazon and have it delivered. We never expected that you could not find it in most of the countries. In some counties they only sell it at pharmacies. Our guides were wonderful. Our guides went to 20 different places in three counties to try to find distilled water. They finally found it for us in Italy...
Be sure to tip the guides well since they go way beyond...
Get trip insurance.
Make sure you do the extra activities. My wife missed a few that we went on and she was disappointed.
Be prepared since European hotels are not the same as US. It seemed to be a common occurrence to have issues with hot water. It might have just been bad luck. Our guides went as far as contacting the utility companies to try to get someone to fix the issues....The hotels were in great locations and where high end.
Consider buying some Euros and Sterling now since the exchange rate is so low as long as you know you are going. It could go lower but the rates are great right now.
Make sure you get a credit card that does not charge you foreign transaction fees.
If you do not need to take calls while you are in Europe then call your cell phone provider to block calls. If someone calls you and leaves a message you will be charged like 25 cents per minute. Typically you can still use internet and get text messages.
If you need to be able to take calls then get a one month all inclusive international pass. Be sure to have it canceled before your new cycle starts or you will be charged multiple months.
Go with the flow because things always happen on trips. Even though I had an international plan I could only call two people in my contacts in some countries. So in England and France we had to call a family member in NH who then would conference call our guides in England/France who were in the same hotel when we had issues. Really strange but it worked...
Again your guides are there to make your trip wonderful so definitely treat them well and utilize them whenever possible.