A very close friend of ours is a Master Plumber and he harps on us all the time about putting food down the disposal. He says disposals were not created for total food disposal and that it is specifically for the remnants that make is down there and clog up your sink. That being said, here are a couple of things I have discovered that keep the sink smelling good. Try orange peels, for some reason, I have had better luck with oranges over lemons. There are also little disposal balls you can get at the grocery store that you put down your disposal every few days to keep it smelling fresh (wish I could remember the name of them, but they are pretty easy to find).
I have used Drano before too, that will help push whatever is stuck down there through (don't tell my plumber friend, he doesn't like that stuff either). Also, if you stop putting food down your drain, whatever is down there will eventually decompose and go away.
Hope that helps. Good luck