The best way to do it, Glenn, is to do what Garnier says.

You're playing NTW at the mo', right? So that means crusades are out of it. However, a quick way to lose influence is to withdraw before an advancing enemy. Giving up some territory to the foe tends to make generals extremely unhappy.

But follow the advised "strip the titles off your best general & then treason trial" route & make sure you give said general (or generals works better) command of a huge stack. That should do the trick.