I've just started using cover crops in the past couple years, so I'm not an expert by any stretch, but I picked up this compost crop mix: http://www.bountifulgardens.org/prodinfo.asp?number=MCO%2D6300 . It includes wheat, rye, vetch and fava. It didn't grow very tall in my garden - maybe two feet tall - so it shouldn't shade out your garden. They recommend planting it at the end of the season when you pull your primary crop, but my growing is so short, I just planted it as the primary crop in one bed. I cut it short a few times during the season, and left the the plant material in the bed as mulch. I cut it the final time at the end of the season very close, and it's all under the snow composting in place right now.
I've seen similar compost/cover crop mixes in other seed catalogs, too. They have slightly different plants, but I think the grain/vetch/legume mix is pretty standard.