Pack half of what you think you need.
See parts of countries that aren’t what you think everyone else is seeing – for example, everyone else is going to Amsterdam, visit Maastricht.
Some of the tourist stuff is that way for a reason, don’t avoid things just because they are touristy. Van Gogh and Rijksmuseum are both well worth seeing, IMO, even though you’ll wait in line with lots of tourists.
Dress in layers. Europe is generally cooler than the USA, but when I was there for the WC (Netherlands, Belgium, Germany and Switzerland were visited) it was hotter than all get out. Since A/C is not as common, you’ll have to plan for both hot and cool weather in the summer.
Back to museums, find out what days are late hours and try to visit on hours that are not normally available, the crowds will be lighter. I think the Louvre in Paris is open late on Wednesdays, for example.
Many/most European hotels and inns offer breakfast, often included in the price. Fill up on this and you can save some money on lunches and snacks.
You will walk a lot.
You may be on trains a lot, particularly if you are not staying in one place for a long time.
Stay in one place for a long time, or at least a while. Don’t move around too much. (I violate this one, personally, but I know Europe pretty well, so it’s not a big deal for me)
Check out www.hrs.de <-- I used this site to book rooms during the WC and got good rates throughout Germany in hotels that were affordable and nice, although not extravagant.
If you’re going outside of the Netherlands it’s probably good to learn a couple of words in the local language, like please and thank you and such. Always good to be polite. “Enter on pain of death” is probably NOT a useful phrase, though.
Pack light. Only pack half of what you think you need. Then take out half and leave it behind. Lugging stuff on a train or through a city is no fun. Figure out how to use local laundromats, and don’t be shy about wearing an outfit twice or six times before you launder it.
I’m not female but I imagine heals suck on cobblestones. Go with comfort over style if forced to choose.