I got some of these for 2 of my controllers, and they're just awesome. You clip them on the existing R2 and L2 buttons, they prevent you from slipping off the button easily, f.ex. during racing games I often slipped of the R2 button (acceleration) which makes me loose precious time.
They are very cheap, about 5$ the pair, and they're also well made. They didn't fall off or such one single time, they're pretty stable. Might be interesting for some of you.
Here is also a video where you can see how it works.