What I tell people who ask me for advice is "you will be the same person in Europe that your are here at home". What does that mean?
If you wouldn't wear a piece of clothing here at home, you won't want to wear it in Europe (There is an excellent post above that makes that point very well. For me it is scarves. Many people recommend taking them to dress up your look, I don't wear them at home, so I know I won't wear them in Europe)
If you don't love visiting every church and museum you can at home, don't try to do it abroad. I usually pick one per city. (I've been to London multiple times and still have world class museums I need to visit..next trip) My first trip I packed it full of churches and museum and quickly burned out. I also found that I had not included enough outdoor activities. (here at home I hike or bike at least weekly) Now my trips reflect what I enjoy doing more accurately.
Know what your travel pace is and have your trip reflect it. After reading this board for several years, I know I travel at a faster pace than many. I am just a get up and get going kind of person. My pace works for me (and luckily for me, it is my husband's pace as well) I am very glad I had already visited Europe on many trips before reading this board because I may have "slowed" my trip down to a speed that would not have worked for me. How do you travel here in the states? I personally would not try to see Washington DC, Philly, New York and Boston in 7 days. (There are people who post those schedules using European cities!!) However, I also wouldn't plan on allowing 2 days to "soak up the atmosphere" of D.C My schedule would have me doing "something" every day while allowing a few hours for relaxation and exploration. That is what works for me. I also do what Brad does and visit regional areas thoroughly versus hop skipping around Europe. That is what I enjoy most.
"To thine own self be true" and have a great trip.