The Nuggets do it on a game-by-game basis... they play hard against good teams and then barely try against the bottom-feeders.
But the Nuggets lacking mental discipline? I don't think that's a surprise to anybody after looking at their roster...
Of course you could make the argument they haven't won a championship while employing their little "on/off switch", I think last year they gave it as good a shot as anybody else. Besides, not all good teams win it all (think of that Mavericks team that lost in the Finals).