People in France have known about raised beds, and used them successfully, for many years. Their method began to be used in the United States in the early 1970s when a group called Ecology Action started a garden in California. It's called the French biointensive method, and you can use its principles to improve your growing areas by implementing some simple changes to your existing beds. In more recent years, a similar method called lasagna gardening has become popular, and it's even easier.
The Biointensive Method
Measure your planned garden area and mark the area with stakes and string or simply sprinkle flour on the ground. If you are transforming an existing garden area, you must first remove all existing plants, especially weeds. A good size for raised beds is 3 feet wide by 6 feet long---you'll be able to reach into the center of your bed without stepping on the soil, which will compact it.
Set a sprinkler in the center of your bed and let it soak the area well for at least one hour. Then wait 24 hours while the soil soaks up the moisture. Cover the area with a 2- to 3-inch layer of compost, which can be well-rotted manure or composted leaves and other plant parts.
Start at one end of the bed and dig a trench with your shovel, making it one shovelhead wide and one shovelhead deep. Move the soil you dig out into a wheelbarrow---you'll be using it later at the other end of your bed.
Dig one more shovelhead deep in the trench you just dug. Then move to the adjacent area and dig out one shovelhead deep as you did before, but move this soil to fill in the first trench. Continue "double digging" until you reach the end of your bed and then fill in the final trench with the soil you moved to your wheelbarrow.
Pile up organic materials such as grass clippings, composted leaves or manure, peat moss, wood ash, blood or bone meal on top of your planting area. The more you use, the richer the soil will become. Then mix in these materials with a spading fork. You can plant immediately.
The Lasagna Method
Measure and mark your beds to be about 3 feet wide by 6 or 8 feet long.
Mow or whack all weeds and existing plants---there's no need to pull them out by their roots or to dig out old lawn.
Lay flattened cardboard or a thick layer of newspaper on top of the soil. Make sure not to leave any areas where the soil is exposed.
Spread layers of organic materials on top of the cardboard or newspaper. You can use any type of compost, peat moss, wood ash, grass clippings, manure, topsoil, chopped up plant parts such as last summer's corn stalks, or whatever you have available.
Water your raised bed and plant immediately by digging through the layers deeply enough to accommodate the root systems of the plants you are planting.