Ffs, i had the same problem on my DSlite and only after like 3 months of using a DSi it starts to do the same. Blowing in it is retarded since after a while you need to blow in it again. Are there any more long-term solutions?
Yes there is a long term. Edit: actualy it's not really long term as it can happen again if you let your DSi get dirty
I had that with my R button (i have a DSi black)
So I opened it up... and cleaned both my L and R button with alcohol and put back in
The awesome thing is that you only need a regular small screwdriver to open your DSi (no triwing at all)
The second awesome thing is that you don't void your warranty as you don't mess with the sticker on the bottom and you don't modify anything!!
Funny thing that happened is that when i opened the battery case i saw those tiny screw like thingy on the top of the battery and I though they were suposed to be removed but actualy they are what controls the voltage on the bottom and top screen. Luckily I only messed with the bottom screen and it gave me quite a scare but now it works fine. Those screws works the same way as if you tune your TV atenna for better reception (gets all fuzy)
I wish I could take pics and make a tutorial but my only camera being the DSi
I cannot take pictures with it while taking it appart xD