Forgot that one
Anyway, too lazy to see if this has been said already so excuse a repeat, but Sheffield motivates himself by getting angry at people and threatening them. Increase in motivation = increase in playing performance, so it's just how you have to deal with it. The Yanks are used to it, and as long as they get the performance out of Sheff, they'll be able to put up with his outbursts.
Also though, Sheff, if you wanna get those 500 HRs, you shouldn't suck at the plate just to piss off the Yanks.