DO - pack light. Originally, I was very overwhelmed by the prospect of packing for a six week "vacation" in three cities with a variety of climates. I ended up committing to a capsule wardrobe concept - 25 items of clothing to mix and match for multiple outfits. I looked at this post for my inspiration:
But, aside from that, it has been working out really well. Mike went with an even more compact wardrobe. I think he has six shirts, two pants, and two shorts.
At the same time, DON'T be unprepared. I followed European weather for a couple of weeks to give me an idea of what I oould expect. Had I just packed for "July", I would have had nothing but shorts and tanks, with a couple of short sleeved shirts for good measure. Instead, I am wearing a lot of pants and the occasinoal sweater. I have an umbrella and raincoat - things I never need in Las Vegas.
DO remember the creature comforts. There are certain things that will make you happy, no matter where you are. Everyone will have a bad travel day. Your flight will be delayed, you will get lost in the streets of Venice, someone will be mean to you in the museum... whatever it is, your day will not go as planned. It is so much easier to get your head back on straight if you have some sort of security blanket, either literal or metaphorical. Mike has an actual blanket, I have episodes of Law and Order (Jerry Orbach years) downloaded onto my Mac. Nothing makes a terrible day better than relaxing for an hour with Law and Order. For you, it is probably different -- a favorite movie, a good book, your favorite chocolate bar.... but be sure you have something to comfort you when the day goes poorly. I loaded my iPhone with a bunch of new music (Amazon Prime now has a free music service) so I could have a new soundtrack for wandering the streets of Paris. I have songs that make me happy and upbeat for all those miles of hiking I do every day. I also have guilty pleasures, so I can sit on the subway and listen to Nickelback's top 20 (don't judge me). These little things make a good trip great.
However, DON'T be afraid to try new things. Delicious foods, a new route to the subway stop, a different kind of coffee or soda. Something new can become your new favorite. Soda is my favorite. I do not drink a lot of it, but I like to try new soda in foreign countries. France has delicious flavors -- Peach, Grapefruit, Green Apple (they like fruit flavored, obviously). Chips are also fun. Mike has prided himself on trying every bizarre type of chips he can find. He likes roasted chicken and bacon flavors a lot (those are two things), olive is not what we hoped it would be.
DO go off the beaten path. Every city has its tourist must-dos... but do not define your trip by these things. Some of my greatest experiences are the random experiences that I stumble across. I ate a delicious crepe while sitting in a park watching the people go by. This was a wonderful moment that you will not find in any guide book. Try the small museums, visit restaurants without an English menu, wander through the small shops, buy chocolates from the supermarket -- Unless you are in China, then I recommend avoiding the supermarket chocolate at all costs.
But, DON'T be afraid to be a tourist. Rick Steves writes great European guide books. He tells you about all of the little nooks and cranies of the cities to make your vacation wonderful. However, he is adamant against being too touristy. He hates the Latin Quarter because it is not local enough - it exists only to serve tourists. But, he misses the point - you are a tourist. Yes, I have wonderful local experiences, but I have also had wonderful French meals for 10 Euros because of the tourist competition in the Latin Quarter. Buy souvineers. Buy subtle reminders of your trip, but do not forget to get the obligitory Eiffel Tower statue. People will ask to see it.
Last but not least, DO follow your own rules. I see a lot of people wandering through the city, using their Rick Steves guidebook as a Bible. They follow the maps, use the pre-determined routes and eat at the recommended restaurants. Sure, you will have a good experience, but you will have that experience and not your own. I could make a list of every incredible place I have been, but then you will simply have my vacation and not your own. Make your own way, go at your own pace, and create your own memories. They will be so much more valuable.