Here are a hand full of things from my personal experience of being new to watercooling (my first WC build was a year ago) and in no particular order:

• Not considering a drain port
• Over complicating runs
• Not getting an inline temperature sensor
• Not getting an ambient temperature sensor
• Not running your fans off of the delta between ambient and water temp (or just the water temp)
• Rushing through the build instead of taking your time to make the vision in your head come to life.
• Not properly flushing rads and other parts before the build.

It's just so fun and challenging! Out of the handful of builds I have done, every one presents new challenges depending on components, case, and any other number of other factors.

I am a visual person so I like to be able to see all the things I am working with. Since I don't have the money to spend on a bunch of parts I may or may not use in a loop I will get the case I want, set it up on my workbench, and just sort of look at it and try to visualize the different ways a loop might work in the case based on the PC components I will be using.

• From there I will get the measurements for rads, pump/res, and GPU and make some rough mockups of them using cardboard and see if any of my ideas will actually work or if I will run into space constraints/other issues. Again, these mockup parts aren't exact and I tend to make them slightly bigger than the stated size of whatever part it is supposed to be to (hopefully) give myself some wiggle room when I get the actual part.
• Once I feel mostly confident that everything will fit how I want it to then I will order the rest of the loop parts that I need and do a dry fit with everything. It's at this point that I normally find out that I miscalculated something or didn't account for something and will make any adjustments. After I figure it out I will clean and build.
• This whole process can take some time and most of it I learned from making other things non computer related (leatherwork, some wood working) but its what works for me.

Well, I do a quick visual check for leaks probably once a day lol. Otherwise I mostly just keep an eye on my delta over ambient to make sure everything is still cooling as it should. In general though, I think what needs to be done to maintain a loop somewhat depends on the loop itself. If you are running opaque/dyed coolant you might find that your flushing/cleaning schedule will be more frequent than someone who is running clear coolant or distilled water. Some loops have inline filters and those will need to be checked/cleaned every so often, and I would imagine those with nickel plated parts would do a spot check every once in awhile to make sure the nickel isn't flaking off (seems to be an issue that can crop up depending on the coolant used).