I have been working with a mother of a toddler, now 21 months, who has
had repeated plugged ducts since he was 9 months old - none before that.
 They are always in the right upper outer quadrant (he is now only nursing on
the right side), generally are painful, take up to 48 hours to resolve, and
have, to her knowledge, never progressed to mastitis.
     I first spoke to her  a month or so ago.  At that time, she had tried
most of the usual interventions for plugged ducts, was fairly knowledgeable,
and stated that the usual heat, massage, etc., had never been helpful for
her.  She states that it usually seems to happen when he's doing the toddler
thing of on-and-off, distracted, interrupted nursings.  She is very careful
to stop him and start over if he is not in a good position/latch.
     At that time, I recommended eliminating saturated fats and taking
lecithin as per Ruth Lawrence, etc.   She called back