M.G.
Vinegar and Baking soda work perfectly. If you can take the rubber thing off that's right at your sink it's a little easier, but you don't have to. Just pour baking soda and then pour a little vinegar and let it sit for a little while. Don't put too much vinegar, though. Just enough that you can hear it bubbling. Then after a few minutes, just run some hot water, and you may even want to run the disposal.
FYI We used to use lemons (and we still put them down there anytime we have them), but they really only mask the odor for a brief period of time.