How do you reset the cache on the Xbox 360?

Go to the system blade and then select memory.

Press Y on the HD symbol and then press X,X,Left Bumper, Right Bumper, X,X.

Once you do this a messsage will appear saying: "Do you want to perform maintenance on your Xbox 360 storage devices?"

Note: this will also remove any game updates you have installed, so next time you load up the game you will have to re-apply the updates.