A lot of these have already been mentioned but my favorites -
-plot anything of interest on a map. Not just must-see sights but restaurants I read about, cafes, bars, small sights that are interesting but not essential, everything. Add a quick note about why it's interesting. Keep the map on my phone (I use an app called citymaps2go, but there are plenty of other ways to do this). Then if I'm in an area with time to kill I can look at the map and see that I'm near that church that sounded kinda cool, or that bakery with the famous pies. Things I wouldn't necessarily seek out but since I'm there will make my hour to kill more memorable.
-before I get there, use google street view to "walk" around the neighborhood. Helps with a sense of familiarity and sometimes I see things I didn't know about nearby, so can look up and mark on the map (see above)
-If you have the kindle app (free), it comes with an email address that you can send files to and they show up in the app as PDFs. I use this for all sorts of things. Hotel confirmations. Guides of highlights that you find on art museum websites. Maps from tourist bureau sights. Bus schedules. Backups of my train tickets (can re-print from here if something is lost). My own notes. Anything, really. Then you can access this info anywhere without lugging around paper. If somehow the app is lost or deleted, the documents are in my account and can be re-downloaded on another device.
-GPS on smartphones is not dependent on having data. If you don't want to pay for a data plan and turn data off, you can still look maps that you downloaded from many apps and see a blue dot marking your location that moves in real time. This is a lifesaver when lost in places with no street signs - you can see which way you are going and which alleys go where, which in warrens of twisty streets, is much harder than it sounds.
-Want to see famous things uncrowded? Go as early as possible. If it's a sight with entry times, be there before the doors open and be the first one in. If it's an outdoor sight that is open all the time, be there as close to sunrise as you can. No one is at the Trevi Fountain at 6AM, but by 9, maybe earlier, until all the way past midnight it's insane. Most tourists don't want to get up that early, and those on bus tours staying outside the city, or day trippers, can't be there that early. It's you and the street sweepers, with the best lighting for photographs!
-Also stay out late. Evening walks are great, long, late dinners are great. So...up early, out late - what gives? The siesta exists for a reason! An afternoon rest gets you away from the worst crowds and lets you rest so you can be out at more fun times. Early and late.
-Plan 1 or at most 2 important things per day - this includes morning, afternoon, and night. So maybe my plan is "Art museum morning, wander and explore afternoon, dinner reservation at FancyRestaurantX". I'll do more than that, but there's flexibility - during "wander and explore", I'll pop into sights, eat something interesting for lunch, and so forth, but there's nothing I HAVE to rush around for, nothing scheduled between the morning museum and the evening dinner.
-Lastly, for popular sights that I actively want to see I make sure to book tickets ahead if at all possible. Sometimes I just don't care that much, but I'll go to see how the line is. Sometimes the going early plan works and I get right in, but sometimes it's just too long and I decide it is not worth it. Which is fine - but if I really care to see it I will book ahead to make sure this does not happen.