My aunt just had a plumber guy over to replace the disposal. Of course, he had to try to fix it first as part of the warranty policy, but wound up replacing it entirely.
He said about once a month, grind up about a half a bag of ice. This cleans and helps sharpen the blade. I watched him do that, and after about 2 minutes of the water filling sink up, all of sudden this brown yucky mess started coming up out of the drain. Then it suddenly all started to go back down the way it's supposed to. He did it a second time with a little less ice, and only a little bit of brown came up before going back down.
So get a sink full of ice, turn the cold water on, and let it run. Afterwards, toss down an orange or lemon and let it do it's thing.
If I have fish or something that requires a bit of lemon juice, I use the left over half to clean the countertops, sink, and stove. Then chuck it into the disposal.