Another way to use a small space is called inter-cropping. The idea is to plant late maturing crops right next to early crops. The early crop is harvested, the plants taken out, and the late crop takes over.
A friend perfected inter-cropping. He presented his garden at a meeting, and a government guy wouldn't believe he could get that much production from his sized plot. It does takes a bit of research to figure which crops will inter-crop. I can't remember much, but I think peas (on poles) were one of the early crops.
My friend also harvested Jalipano and Big Jim peppers in Ontario, Canada by starting the seeds indoors during January. Starting seeds indoors is a way of growing crops in areas where the growing season isn't long enough. The idea also can get more production out of a small plot. Start and early crop indoors, harvest it, and plant another early crop. However, sometimes the second crop won't taste real good if the weather gets hot.
My friend and his wife did spend a lot of time gardening, and it's surprising what can be gotten from a small plot--sometimes even surprising the gardener. He reportedly stormed out of the house one evening after dinner and ripped out all the zucchini plants. Said that he had eaten zucchini in some form two meals a day for several weeks, and there was 20 lbs. or so in the freezer. The plants just wouldn't stop, and he couldn't take it any more.