For a great how-to on planting and caring for cover crops go to:
http://www.organicgardening.com/feature/0,7518,s1-3-81-789-1-1-2,00.html
The only thing I don't agree with in this article is the tilling. However, until I can find a great way to use a perennial cover crop (which is High Risk) you will have to till it to make sure it dies. The annual cover crop (Medium Risk) you can kill by cutting it at the base of the plant to make sure it dies if the winter does not kill it.
High Risk - Cereal Rye: I will not have this available at the garden center because I don't think that many people are going to want to try it this year. However, if you do want to try it let me know and I can help you locate it or I can get some in for you. Cereal rye can be sown right up until frost. The rate I will have to find out if you are interested.
Medium Risk - Annual Ryegrass: We will have this available soon. I will post it here when I get it in. It will be $1.99 per pound. The rate is 6-8 pounds per 1,000 sqaure feet and should be sown in August or September. If your beds are not empty by September you can try a mulch instead or just sow around the plants. In my mom's garden I am going to have her sow around the existing plants and cut or pull the existing plants when the are finally finished.
Low Risk - Mulch: This is not a true "cover crop" but still provides many other benefits. It is always beneficial to keep your garden covered up. For the mulch you can use: grass clippings, chopped leaves and straw to name a few. When you clear out your beds after harvest just cover them with a mulch.
If you need further assistance please contact me!
kenny@deneweths.com
or call Deneweths Garden Center at (586) 247-5533 and ask for me, Kenny.