For those of you with squeaky 3DS hinges, use some lube.
Seriously, it shouldn't be harder than that, just make sure you apply it with something thin, a piece of paper should do the trick
just to make sure you get enough lube where it's needed.
My 3ds d-pad was squeaky when I first got it (early April) but after lubricating it it got smoother and hasn't made any squeaky sounds since. Thankfully that's the only fault my 3DS has, the hinge is slightly less loose than the fist video example, and so far there hasn't been any kind of marks on the top screen left by the bottom part, well except for one time when the circle-pad was dirty but it was only visible if you held it under strong light, nothing I couldn't get rid of with a microfiber cloth though.
And yes I do take my 3DS out so it isn't like it's always zero pressure on the lid.
For those with scratches on their screen or whatever part of the 3ds that's been scratched (since all the 3DS's parts are shiny plastic) use DISPLEX, with time and some elbow grease that thing does miracles. I've actually already used it on my 3DS bottom part (battery cover) which had barely visible scratches but still it was rather new and it was before I got some form of protection for it.
Anyway...
Is it absolutely necessary to write the serial #?
I have thin protective aluminum caps on the bottom and top of my 3DS, and I'm I don't feel like taking them off right now but if you really need it then...