my "R" button stopped working after a time. I had to press on it so hard to make it work that my finger would hurt. nintendo told me that my warrenty was up and it would cost me $86.50 with shipping to have it looked at and fixed. obviously i wasn't about to spend that on a 2yr old ds with scratches on the touch screen, so i used a can of compressed air. the "R" button sits just by the power area and if you pull it back far enough there is a small hole between the button and the case. you can get some good air squirts in there and really clean it out. after a few tries i got it all clean and it works perfectly.
i did a bit of research around on various forums and it appears that the micro switches used for the shoulder buttons can be clogged with tiny bits of dust and dirt. to think that nintendo would probably have done the same thing. also if blowing in there with the case connected just doesn't seem to do it, and you're no longer under warrenty you can pick up a triwing screwdriver for around .99-2.00. there are a few guides to opening the case of the nds. just take your time, but you will find when its open that it is 100x easier to clean out dust and dirt.
goodluck,
-another world