Two cover crops that I have used successfully are Austrian Winter Peas and Winter Rye.
The winter rye --not annual or perennial you want it to grow in the winter not die-- is best when planted in September but I have had success with later plantings(as long as it is warm enough to germinate). It is used mainly to improve clay soils. Broadcast the seed, water, and let it grow. How much seed you use is up to you. I guess about a pound would seed @200 sq.feet. Sometimes, if you have planted early it may need a hair cut if you want a neater look. Let the cuttings lie. This rye will die in the heat of late spring(June) so you can plant in it and let it act as a mulch or turn it under into your bed. The root system is extensive and deep so I chose, after trying all ways, to pull it out after it dies and add to my compost pile. It is not hard to pull after it dies.
Austrian Winter Peas are my cover crop of choice. You should use an inoculant when planting these. The inoculant introduces beneficial bacteria to ensure the formation of high-nitrogen nodules on the roots. You can broadcast or plant in rows in your beds. Cover with a thin layer of soil to deter the birds from eating your seeds. They continue to grow and add biomass in the winter(slowing as it freezes). When the pea tips form pinch them and add to your winter salads. That's an added benefit to using this cover crop. They are yummy. Like other legumes,winter peas add nitrogen to the soil so they help rejuvenate your beds. In spring or whenever you are ready to use the bed, either turn them under as green manure or pull and add to your compost pile. They are easy to pull. Just a note: I plant winter peas OVER my garlic bed so I have two things growing in one bed.(The peas also keep the weeds down.) The peas are gone before you dig the garlic BUT if not, as you dig the garlic you can turn under the pea biomass. If you plant other winter root crops you may try the peas over them as well.
Sources for these cover crops:Peaceful Valley Farms @ http://www.groworganic.com/ has the Austrian winter peas and inoculant. Seeds of Change also carry the peas. The seed is less per pound at Peaceful Valley but shipping is higher than Seeds of Change. Johnny's Selected Seeds @ http://www.johnnyseeds.com/ carries the Winter Rye. I have used Abruzzi Rye which is carried by Southern Exposure Seed Exchange . Locally Southern States and Ashland Seed carry the inoculant. You want to ask for Garden inoculant or one for legumes.
You can click here to watch a short YouTube video of local sustainable agriculture professor Cindy Conner talking about cover crops in promotion of her DVD.