First of all, I don't know if this is REALLY really reliable (A.K.A. that does work to have it fixed forever), but as far as I have tested it, it does work way more time than when I tried the classic "blowing" method.

As I have no idea how to disassemble my NDSL, I did try recently using a thin stick to get the dirt off the gaps in the shoulder buttons of my NDSL, which had a unresponsive shoulder button problem...and it seems to work fully responsively and constantly!

Simply grab a thin stick or something thin to clean the shoulder button gaps. I used one of these: https://http2.mlstatic.com/palitos-de-altura-30-cm-D_NQ_NP_767305-MLV20847375612_082016-F.jpg

Then use the thin stick, and keep brushing the gaps off for a while, and then check if it's now working.
I can confirm for myself (at least) that it does give a good fix to the shoulders! Just was trying with Mario Kart DS, and works pretty well.

Hopefully this gets work for everyone else!

 

That's when you use a good ol' can of compressed air...

 

thats really not doing much, the only thing 'blowing' into it fixes is get it humid, making it easier to close the circuit to send the 'button pressed' signal.
its not really cleaning anything. you need a cotton swab and alcohol to actually fix this.
at least thats how it is for the dsphat.

the dslite seemed to have most of its problems caused by switches being pushed in too hard to the point where clicking the button down would not close the circuit mechanically as it should. pushing down even harder sometimes still made it work. there, humidity too would probably help too up to a certain point, though its even more of a temporary fix as it was on the phat.

 

The shoulder buttons are actuating a microswitch. The best fix is disassembling the shell and blasting those micro switches with contact cleaner.