Garbage disposers clean themselves if used properly. Do not put lye or other chemical drain pipe cleaners into the disposer.
Periodically, and always after disposing of fibrous food wastes, purge the drainline by filling the sink with two or three inches of cold water. Turn on the food waste disposer and allow this water to run through with no wastes added.
Before leaving your home for several days, check to be sure all wastes have been flushed out of the disposer to avoid the creation of odors. If odors should occur through an oversight, they can be removed by running orange or lemon peels or ice cubes through the disposer.
[Info from the Home Maintenance And Repair Database at the Michigan State University website]
Citrus peels are great also you could try pouring baking soda in the drain then pour vinegar over the baking soda and wait a few minutes before rinsing.
Pressurized drain cleaning products, like CLR Power Plumber, offer an alternative to chemical drain cleaning. One of the CLR products is usable 10 or more times. The 4.5 oz replacement cans can be found at Amazon.com for around $8. The actual pumping action product is usually priced at around $30. Plumb-Away is a similarly operated and priced device, also able to be found on Amazon.com.
They generally advertise the pressure can clean:
50' down a 1.5" diameter pipe
40' down a 2" diameter pipe
35' down a 3.5" diameter pipe
25' down a 4" diameter pipe
18' down a 5" diameter pipe
A quick, cursory search on Google for either product name, in quotations, will yield many forums of happy customers.
According to the author of Talking Dirty with the Queen of Clean, Linda Cobb, fill the sink with 3 inches of warm water and mix in 1 cup of baking soda. Drain with the disposal running. This should keep disposals fresh and free-flowing.
I've found the best way to clean it is to run the hot water tap until the water is hot, then reduce the flow to a trickle and turn on disposal. While the hot water and the disposal are on, add a big squirt of liquid dish detergent (NOT automatic dishwasher detergent) down the disposal, and let both of them run until all the soap has been flushed away.
The hot water and the motion of the disposal "whips" the detergent into a frothy suds, which cleans not only the disposal, but the underside of the rubber guard in the opening of the disposal, as well. It also helps by the suds 'floating' up food particles such as celery stalk strings that have wrapped around the grinders, then flushing them away.