Autumn is a good time to work into the soil any compost you have made over the summer. Or, you can be lazy (as I often am) and just spread the compost on top of the soil, letting the worms do the work. If you haven't made any compost, buy some for immediate use, and start a compost pile for next year. You can also shred leaves with a lawn mower (if you have one) and mix the shredded leaves and grass clippings into the soil. They will break down over the winter.
While you're working in organic matter, you could also add some organic fertilizer. You can buy pre-mixed ingredients or separate components. Try adding small quantities of fishmeal (for nitrogen), bonemeal (for phosphate), and kelp meal (for potassium and trace minerals).
Here are a couple of sources for winter cover crop seed:
Territorial Seed Company
PO Box 157
Cottage Grove, OR 97424
USA
telephone 541-942-9547
fax 888-657-3131
Johnny's Selected Seeds
Foss Hill Road
Albion, ME 04910
USA
telephone
fax 800-437-4290
fax outside USA 1-207-437-2165
email: homegarden@johnnyseeds.com