How can I press buttons faster?

Some games have segments that require the player to press one button really fast in order to pass through. At the moment, the game that bugs me is the Legendary Starfy, but I've also run into it in Mario & Luigi: Bowser's Inside Story. (What is it with DS games and the "push a button really fast" mechanic?) For the final boss battle in Starfy, the advice is:

Once Starfy (Or Starly if in two player mode) defeats him, he will blast the moon of Bunnera at Starfy. The player must rapidly press Y to deflect it. He'll blast it again, and this time it's even harder. The best strategy is to press Y until the player is tired out, then pause the game, take a break, and do it again.

Besides taking out a contract on the developer who implemented this or hiring a neighborhood kid with fast fingers to finish the game for me, what can I do to press Y rapidly without developing debilitating RSI in my hands?

The first tip I would give you might be obvious, but nevertheless many seem to forget it: Use your index/middle finger instead of your thumb. You'll find that if you place the controller on a surface and tap the buttons with your index finger, you achieve a much faster pressing rate.

The second tip comes straight from the pro, Toshiyuki Takahashi (a.k.a. Takahashi Meijin, a.k.a. 16 Shot), that tip is to use you middle/index finger and your thumb together in a downward vibrating motion, see the following video:

As any rapid, repeated motion, it can cause damage over time, so give your fingers and palms a rest once in a while...

I agree that it is a foolish and arbitrary mechanic that doesn't make any game more enjoyable.

The first and easiest way if the game allows and sometimes even if it doesn't would be cheat. If you don't have to struggle through a rapid-push mechanic, then you don't need to press the buttons faster. Some mechanisms involve software or hacked saves or in more extreme cases even hardware modification. You could try using some sort of physical device outside of your DS like chattering teeth to press the button for you, but these usually prove unreliable unless you've designed some sort of custom controller overlay device that contains a rapid auto-fire as one would find in certain 3rd party contollers for certain consoles.

Barring avoidance of the mechanic entirely, the second method would be to actually physically press very rapidly. This is likely RSI inducing no matter how you go about it. As Mr Smooth suggests, shivering or twitching your entire hand with your finger extended to the button in question can cause your finger to rapidly press the button. In general, you'll likely end up moving your entire hand.

There is a greater problem as some games not only use this mechanic, but make it exceptionally harder by necessitating that it be rapid tapping of alternating buttons. For this, you'll essentially need to perform the same mechanic on two hands and you have to do so in a deliberate way so as to properly alternate between one button and another so this means that arbitrary shivering and twitching are not useful.