I read a bunch of articles on this topic some time ago. Apparently, it involves
not maintaining strict and inbred lines and cultivars, but rather maintaining a genetically diverse blend from a given species across generations (letting them mix freely). Over time, the plants should adapt well to your tastes and your growing conditions, while staying diverse and non-homogeneous. There's selection involved, but unlike line-breeding and inbreeding, you're not looking for a hyper-homogeneous population. Rather than strongly selecting in favor of specific traits, it looks more like a moderate selection pressure against overtly negative traits. Positive traits are selected for, but neutral ones aren't selected against just for being un-ideal. Poorly growing plants are eliminated, and poorly-flavored produce is not re-planted. Positive types are favored, but not isolated (the difference from line-breeding). So it's like selective breeding, but without isolating the result ('cause new genetic material could improve it further, and maintain its adaptability and health).
Here's the link to the collection of articles:
http://www.motherearthnews.com/search?tags=%22landrace%20gardening%22It sounds like a good way to grow strong, vigorous, tasty produce. Basically, mix it all, let genetics take over, then pick a variety from the best. The author seems to get great results with it.