Ogburn's advice is aimed at Oakland, California, residents, but most of the advice is just as good for residents of other cities. Basically it's a matter of finding out who owns the land and leaning on the right people to get permission to garden there. Vacant lot owners may be worried about liability issues with a bunch of folks tromping around on their property, so a certain amount of finesse (and plenty of urban gardening know-how) will come in handy.
[Via SFGate]
Image source: Flickr member sflovestory under Creative Commons