If you're planting your vegetables strictly according to instructions on seed packets or in most gardening books, you're wasting a lot of garden space.
Instructions typically call for long rows of single vegetables with enough space -- one to three feet -- for you to walk between the rows hoeing or rototilling weeds.
The first thing to do to wring more vegetables from less space is to plant in beds. Plant closely in three-foot-wide beds, allowing paths 18 to 24 inches wide between beds rather than between rows of vegetables in the beds.
Experiment to arrive at optimum spacings in your beds, balancing out good yields with sufficient size. Broccoli plants generally do well spaced 18 inches apart in all directions, carrots need about three inches, and onions about six inches.
If you want a lot of vegetables from little space, also plan your garden in a third dimension: up. Peas, cucumbers, melons, tomatoes -- all these vining plants can be planted closer together if trellised or staked.
In a single bed, why not plant trellised cucumbers between late plantings of broccoli and kale? Keep lettuce -- which hates hot weather -- growing more happily into summer by planting it in the dappled shade cast by cucumbers on an inclined trellis. Another plus for trellised vegetables is that they stay cleaner than their ground dwelling counterparts.
Besides filling every cubic inch of physical space, let's also fill the fourth dimension: time. Except for brussels sprouts, leeks, and kale, most other vegetables are not in the ground from the very beginning to the end of the season. So there's almost always time to slip two, even three, different vegetables into the same piece of dirt in one season.
Filling these time slots is like doing a jigsaw puzzle. For instance, plant early spinach where you're going to later plant tomatoes. Bush beans that peter out by midsummer could be followed by cabbage transplants.
Try other tricks too, such as pregerminating seeds on paper towels to cut back the time in the garden from planting to harvest. And use started plants for those vegetables that transplant well. Play around with this puzzle, but record your results or you'll repeat some failures and forget some successes.
The more tightly you mesh time and space in your vegetable garden, the more important that site and soil be up to snuff. Vegetables need sun -- at least six hours a day -- and well-drained soil. Also water as needed and use plenty of compost.