Start with the simple. Check for the most common cause of this - grip rubbing on either the throttle housing or the bar end. (They creep either way under normal use). If that's it massage it away from what its rubbing on.
If that isn't it, open the throttle housing just enough to check the cables where they attach to the throttle tube.
If that isn't it, open the throttle housing just enough to check the cables where they attach to the throttle tube.