You probably see them behind stores all of the time--free shipping pallets make great recycled wood for building raised garden beds. If you're using the Mel Bartholomew method of Square Foot Gardening, you will need to confine your garden soil and keep it off of the ground, so the nutrients don't leech out and children and animals don't trample it.
Select sturdy pallets for your project. The easiest ones to break down are the skids used to transport heavy items like concrete or bricks. These are most often available behind home-improvement warehouses.
Plot out the size of your raised bed. The most common size is 4 by 4 feet square, which will allow you to easily reach into the deeper areas of the garden bed. Alternately, create a larger bed, or several four-by-four squares.
Pound stakes into the ground at every corner, and at 12 inch intervals throughout the perimeter of your garden. For a four-by-four garden plot, use 16 wooden support stakes. Pound the support stakes halfway down into the ground, so if they're 12 inches long, 6 inches should remain above the ground. For 18-inch support stakes, leave 9 inches above the ground.
Cut the long pieces of wood that make the surface of the skid away from the frame by cutting straight through the frame, in between the slats. Some of your pieces will still have the framing attached to them. That's OK, as it gives you a surface to screw onto the support stakes.
Screw the wood directly to your support stakes. They'll fit a 4-by-4 foot bed perfectly. Otherwise, trim the pieces down and custom engineer them to fit your desired size.
Line the bottom of your raised bed with cardboard or newspaper to keep the native plants or grasses from growing up through your soil. These materials break down fairly quickly, lasting just long enough to ensure that the grass underneath is smothered long enough for your vegetables to grow.
Fill the bed with a soil mixture. Amend your soil as needed with peat moss, hummus, sand or organic compost. The Square Foot Gardening book gives specific recommendations for soil amendments, so you can customise a mixture that makes the most of your native soil, or completely replace it.
Ask permission from the shopkeeper before taking wooden pallets. Chances are, they'll be happy to get rid of them, but if they already have an agreement with someone else to pick them up, you may be committing a crime by removing them without permission--even if they're out by the dumpster. If you end up coming across rusty nails, be careful that you don't step on them or otherwise impale yourself. If that happens, visit a medical care facility for proper cleaning and repair, which may include a tetanus shot.