In the original SFG book, Mel suggested reflecting sunlight into shady areas to help get sun-loving plants the needed light for healthy growth. If you don’t mind the appearance, one way to bounce more light onto your sun-loving crops is to line the ground around them with aluminum foil.  The foil will act not only as a mulch to keep down weeds, but is said to act also as an insect repellent.  It seems that the light bounced off the foil and back to the underside of the leaves confuses the insects that expect to find a quiet, dark area.  The extra light is said to help many shady locations.  As the plants grow and produce extra leaves, the advantages of the foil are cut down as the amount of light able to reach the ground surface is also reduced.  

Another rather extreme technique is to line a fence along the north side of the garden with foil, from the ground up to a height of 3 or 4 feet.  Or, you could make removable, rigid panels by stapling or gluing aluminum foil onto lightweight, weather-resistant panels.  You might use Styrofoam or some similar material.  You can remove the panels when company comes or if you get tired of looking at them.  You could also use the shiny, white surface of a panel without the aluminum foil.  This is a little extreme, but if you don’t have enough sunlight, it may be worthwhile.  

You can also plant crops that don’t require as much sun in the shadier areas.  These are beets, carrots, cauliflower, Swiss chard, cucumbers, lettuce, onions, parsley, peas, radishes, spinach and winter squash.  These crops require 4-6 hours of sun a day.