Official Squid Grip for Xbox 360. One Kit includes:
SquidGrip is a low-profile stick on grip with a foam padded internal with a rubbery, moisture wicking layer on top. Extremely easy to install, and adds a little "meat" to the controller. SquidGrip provides a firm grip that is soft to the touch and wicks away sweat.
* Will fit all controllers based on the Stock Xbox 360 shape; Battle Beaver, Cinch, Scuf, Etc. with some additional trimming.