Amanda said: Cover crops are a great idea! Typically, you'd sow in the spring and another round in the fall. We've had a late start, so you're not too late ;) Clear the beds of weeds, loosen the soil, and broadcast the seeds so they're thick but not too crowded. Ideally, do this before a rain event. Mow the crops just before they go to seed -- this is important! Give them a few days for the mown parts to wilt, then till them in. You can repeat this in the fall, or plant there in the fall.
As an herbalist, alfalfa, red clover (must be Trifolium pratense) and oats are great options, because you can harvest them! Other options are rye, buckwheat, and peas. You can mix crops, and rotate them. Alfalfa, Clover, and Peas will fix nitrogen.
Good luck!