Several different seed combinations can be used for cover crops. Vetch mixed with annual rye grass or cereal rye provides both a legume for nitrogen-fixing and the rye for building humus. To seed 1000 square feet, combine 1.5 pounds of vetch seed with 2-3 pounds of rye grass seed. Be sure to till the resulting growth under in the spring just as it begins to bloom. Don't let it go to seed and get woody.
Cover crops can be tough to manage if they get too mature before they are tilled in.