Soak the affected breast for around 20 minutes, prior to nursing, in a basin of warm water. A firm, circular massage can be used during the soak and while nursing. Nurse frequently on the affected breast. Don't forget the basics. Proper positioning and attachment is very important in assuring adequate breast drainage.
Some moms have luck using a rather unusual position.
Place your baby on the bed and nurse from above, with your breasts hanging freely. You can also do some breast massage over the area of the plug as your baby is nursing. Obviously, this isn't an everyday position, but it may help with the more stubborn plugged ducts. Try a cabbage compress on the affected area. Apply to the area, using your bra to hold it in place.