It's commonly referred to as cover crops. Fall is the best time to plant a cover crop. I planted winter rye just after I cleaned up the garden. Now, you can buy cover crop seed at farm supply stores or nurseries.
Several types are usually available.
Just be sure to check with your local county extension service to find which one is good for your climate. It's important that you till in your cover crop before it sets seed, when it's in its immature stage. Tilling the green manure into the soil is like adding a super dose of organic fertilizer. And if you planted a cover crop from the legume family, you get an added boost of nitrogen. Now, once you've tilled the garden and leveled the soil out, it's a good idea to let this bed settle.