Lessons Learned ...

• Buy a guide book for the countries you will be visiting and look up the specific cities before you leave, but don't bring the book with you.  It will be extra weight you will regret at some point when you are dragging your suitcase or carry-on around with you.  Do pack a small notebook with you to write down the day's events and sights for you to look up when you get home.  If you can find a city guide book at a local shop while on a tour, do buy one of those because it will more information than a larger guide book.  I bought one for Basel and for Strasbourg because I knew I wasn't going to buy a larger book for Switzerland or France when I was just seeing one city in each country.  I bought the Discovery Channel's Insight Guides for Germany on this trip but was a little disappointed in it because it did not have a lot of the information I was looking for in it when I got home and was working on my scrapbook.  The next year when I took the Danube River cruise, I bought the DK Eyewitness Travel books and was very pleased with those.
• Pack less ... I packed a different sweater for almost every day of the trip, three pairs of jeans, a velvet skirt, and two dressy blouses to wear with it.  As a result, I lugged through the Frankfort airport and onto a train a very large suitcase, a smaller suitcase, and a purse.  I was miserable! I could have just packed one pair of jeans, worn one pair, and then packed fewer sweaters with maybe some lighter turtlenecks to wear underneath or instead of.  The following year when I went on the Danube River cruise I was quite prepared with only one small suitcase and a carry-on.  Just in case I needed extra space, I also packed a folding bag in my suitcase since I was allowed two carry-on bags.
• If you are flying out of a different city than you live, go up the day before your flight to Europe leaves if you can so that you be rested and not as frantic to make a connecting flight.
• Build up your immunity before you leave by taking extra Vitamin C for a week before your flight and for the entire trip.  Between a 10+ hour flight with strangers breathing the same recycled air, and jet lag, you will get sick whether you like it or not.  Protect yourself by carrying an antibacterial gel to use frequently when flying and once there.
• Pack your basic cold remedies ... cough drops, sinus meds, ibuprofen, Theraflu, whatever you use most because it may be difficult to find it when you are there, and when you feel like you've had a large elephant step on you, you aren't going to want to search for a pharmacy.
• Learn more of the language, and don't try to learn basic French because they are insulted when you botch up their language. 
• Buy less.  I came home with more "dust collectors" than I needed.  If you just can't live without something, see if they will ship it for you.  Take more pictures than what you want to buy.
• When buying postcards, buy two of the same.  Write the day's adventures on one and mail it home before the trip is over, and carry the other home with you so that you have a "journal" of sorts when you get home.
• Be brave when it comes to eating locally or eating something you've never had before.  I once could not comprehend the idea of eating the snails that I had crawling in my fish tank back home, but surprise, surprise, when given the opportunity to eat escargot, caviar, and duck, I truly enjoyed them!
• Relax.  Enjoy.  Explore.  Laugh.  Meet people, and make new friends.
• When coming home, especially if you will land late at night or early morning, make hotel reservations near the airport and stay an extra night.  You will be grateful for the rest before catching a connecting flight home, or driving a long distance.