One approach which works....
Taught to me by a boat mechanic: use a ball-peen (sp?) hammer on the locknut (ball-peen hammers have a round ball head in place of the claw found on typical hammers in case you are not familiar). Works for me everytime on my boat and have used technique on other boats as well. Using the ball end of the hammer head, GENTLY tap on a crown of the locknut, in the direction it needs to turn. At first I used to be concerned about damaging the locknut using this approach, but it hasn't caused any harm after all the times I've done it on my own stuffing box. The shock of the hammer blow is enough to break the corrosion "weld." Simple, quick and works everytime.
