During my college years, I traveled to Europe every summer. Back then, I'd start hatching my plans in spring: First I'd determine how much time I could get away for, and then I'd buy a cheap plane ticket to Europe. After that, I'd figure out where I'd actually go.
These days, I'm much more organized. My first move in planning any European trip is to assemble an itinerary. Although it sounds like a chore, it's actually fun: Filling in the blanks between the flight out and the flight home is one of the most pleasurable parts of trip planning.
A good way to start building an itinerary is to brainstorm a wish list of destinations. It's OK to dream, but have a good reason for every stop. Don't visit Casablanca because you liked the movie.
Take the weather into account; for instance, Scandinavia is best in summer. In spring and fall, snow can close alpine trails in Switzerland and northern Italy. While small towns can be dreadful in the off-season, big cities are interesting year-round.
Once you make a list of destinations, group them geographically. Circle your destinations on a map, then figure out the most logical order to visit them. Write down how many days you'd like to spend in each place, and tally it up. Compare that number to how much time you have for your vacation -- then, if necessary, start cutting.
To pare down your itinerary, minimize redundancy. If both the Italian Riviera and the French Riviera are on your list, ax one. Instead of mountain hopping through Switzerland, focus on a single part of the Alps. You don't need to see multiple English university towns. Choose one (I prefer Cambridge over Oxford).
You can also trim time from each stop. Although five days in Paris would be grand, you can see the high points in three. But don't go overboard. One-night stands are hectic. Try for at least two nights per stop. I'm happy to endure long hours on the road or train and hurried sightseeing along the way in order to enjoy the sanity of two nights in the same bed.
It's smart to strike the right balance between intense big cities and laid-back villages. For instance, breaking up a tour of Venice, Florence, and Rome with an easygoing time in Italy's hill towns provides a nice and often necessary breather -- a vacation from your vacation. Plus, judging Italy by Rome alone is like saying you've seen America by only visiting New York City.
Sometimes logistics can help guide your decisions. For instance, on a three-week grand tour of Europe, a stop in Istanbul may take too long. Consider distance and convenience, and eliminate destinations that take too much time and effort to get to.
Making wise transportation choices can also shave time off an itinerary. Instead of spending precious daylight hours on the train, fly or take overnight trains (for example, from Munich to Venice). Flying into one city at the start of your trip and out of another at the end is usually more efficient (and not much more expensive) than booking a round-trip flight. Think carefully about which cities make the most sense as a first stop or a finale.
Another strategy is to settle down in a central location and use that place as a home base for day trips to nearby attractions. For instance, staying in Madrid for a week allows you to make day-trips to Toledo, Segovia, and El Escorial, while enjoying the more exciting big-city nightlife.
Once you have a basic plan, fine-tune your itinerary. Start by anticipating any closed days. For instance, in Paris most big sights are closed on either Monday or Tuesday. It would be a shame to be in Paris only on a Tuesday, when the Louvre is closed. You can also take your trip to the next level by researching special events, such as market days, festivals, concerts, and sporting events. I like to write out a daily itinerary that includes a general list of places I want to hit each day.
Having an itinerary doesn't mean that spontaneity and freedom have to suffer. Although I always begin a trip with a well-thought-out plan, I like to keep some flexibility. Perhaps I'll fall in love with Siena and stay an extra day. An itinerary simply allows me to see that that extra time in Siena will cost me a day in Florence.
It's important to be realistic about how much you can cover. You can't visit every place in one trip, so don't even try. Enjoy what you're seeing, and consider it a blessing that you have a reason to return.