I'm unfortunately prone to plugged ducts and mastitis, and have more than a few tricks that help me. Definitely, nurse as much as possible, rest, and drink plenty of fluids. A plugged duct ideally should mean you act like you have the flu - you're in bed in your pjs with your nursling. Reality doesn't always allow that, but if I do take that approach right away I can usually clear things up quickly.
Nursing with your LO's chin towards the plug helps increase the suck and is more likely to move milk from that area. It can help to lay your LO on the bed on bed on her back, and nurse with your breast hanging down, you on hands/knees, her chin toward the plug. Sort of funny looking and tends not to work with an older nursling who is probably going to laugh and get silly instead of focusing on nursing. Massage has always been critical for me as well; pressure around the sore area as firm as I can stand, moving down toward my nipple. Breast compression while massaging helps as well to get that milk moving. If your nursing isn't willing to nurse more than usual, you might try pumping; even though a pump isn't as effective as a baby at moving milk, sometimes it helps to increase the frequency of milk movement. I've always found it helpful to soak in a hot bath for 20 minutes, then pump while I'm in the tub, taking lots of breaks to massage and try to hand express milk from the plugged area. Cold compresses between nursing sessions for discomfort, warm compresses just before nursing. Ibuprofen for inflammation is typically safe while breastfeeding.
Other things to try: cabbage leaves (often recommended for engorgement, help with any sort of inflammation), antiinflammatory herbs such as marshmallow root for your bath water, if you take herbals then a good echinacea or other immune stimulant such as garlic. One trick that my midwive told me about is a potato compress - grate a raw potato (skin and all), apply to the plugged area, cover with a hot-as-you-can-tolerate wet cloth (microwaving a wet cloth diaper works well), keep it on for 20 minutes or so then nurse or hand express to try and move milk.
If you think it's turning into mastitis, I have had great success w/ Wish Garden's "happy ducts" (used to be called mastitis remedy; I guess the cute name sells better).