An appliance handyman showed me this trick awhile back and it amazes me every time how much gunk comes up.
How to Clean Your Garbage Disposal:
1. Fill the disposal opening and sink with ice.
2. Turn on the disposal.
3. Add a little water.
4. Use the handle end of a screwdriver (not your fingers unless you're a professional!) to push the ice down into the disposal until the ice and water create a swirly slushy.
5. Let it run until all the slushy ice water goes down the disposal drain.
It will be squeaky clean after this trick. Try it for yourself!