I just did this to restore my L/R buttons back to new on my Lite I was repairing. Ordered tri-wing screwdriver, ordered replacement case, did all the work of opening my two broken DS Lites and switching the working screen from one to connect it to the motherboard of the other one, had to deal with taking apart the touch screens to swap the touch panels between both my DSes because I carelessly damaged the touch panel on the functional bottom screen I had (touch panel on the broken screen was fine so I swapped them), had to deal with shoddy quality of replacement case which made it a lot harder than it should have, learned not to put the screws back on until I know everything is working, got pissed because power went out shortly after I began work so I had to halt due to it being too dark, finally finished it hours later once power came back... only for me to try playing a game and remembering that my L/R buttons weren't really working

So I looked up "fix ds lite l r buttons" on Google and this thread was the first result turns out taking apart the shoulder triggers isn't as tedious and easy to mess up as I thought it would be, and it's amazing just how much rust accumulates up in there. Scratching at the metal contact inside the trigger and at the metal plate and being amazed at just how much shinier I can make it just by doing that. Put everything back together and went back in a game, L/R buttons working perfectly now :3

Edit: If you're interested, this is how the metal caps inside my triggers looked like... http://i.imgur.com/NhaDYS2.jpg
They were completely done lol.

 

A much easier, less invasive and equally good working method is to use contact spray on the buttons. Worked perfectly for me on several consoles.