I'm just a hobby gardener now, with maybe 1500 sq ft total that I manage with 5-hp tiller and hand tools. In the past I've had as much as 1/3 acre that required a small tractor and larger tiller. With the larger spot, I burned the cover after a killing frost, cut it in, planted annual rye as a manure crop, and disked that under in the spring. I scrounged all forms of manure, rotted sawdust, etc, and cut that in as well.
With the smaller plot, I compost scraps and all plant debris from the garden and till that in, in the fall. Weeds are kept down during growing season with leaves and newspapers, which are tilled in, in the fall. This year, as a tip from Mother Earth (Oct/Nov 2011), I'm going to plant Durch White clover as a ground cover and let it go as a living mulch in the spring. I scatter a few mustard and turnip seeds at intervals over the old growing area and keep some around most of the year.
I'm not totally organic, I will Roundup pesky weeds when I'm too lazy to run the hoe, but so far they haven't been much of a problem.