There is little doubt that raised beds have many advantages for gardeners.

Some of these are:

• Eaiser to work in, less stooping/bending.
  
• The soil is not walked on so remains looser and uncompacted.
  
• The soil warms faster in spring allowing earlier planting.
  
• Better drainage.
  

Many folks ask what is the best material to use. One very good choice would be to use one of the recycled plastic/wood composite products that are appearing on the market. It has many advantages over other materials. The main ones being it's enviromentaly sound, last's a looooong time, easy to work with, and it's attractive.

These products are becoming more widely available and, while pricey, have the advantage of never needing replacement. Well, maybe not never, but many times longer than natural wood. Easily 10 years and probably much longer. There are also no problems with toxic leaching you have with other material like Pressure Treated lumber. Since the plastic component is generally food grade recyclables and totaly inert.

Better stocked locations will have a full range of dimensional sizes from 2 X 2's on up.

Some brand names are:

Trex

TimberTech

ChoiceDeck

Large retailers like Lowes and Home Depot have also begun to carry some of this material so finding it should be no problem.

For corners you can simply butt the ends onto the corner stake and screw them in. The material holds screws very well. Use a non corrosive screw like stainless steel to ensure long life.

I did these drawings awhile ago and they may give you some ideas. I sized them to use standard dimension material and reduce waste and cutting.

The materials list is:

• 4-16' lengths 5/4" X 6" for side walls
• 1-16' length 5/4" X 6" cut into 4' lengths for ends
• 30-4' balusters for shadow line,top ledge support and cut down to 2' for positioning stakes.
  
• 2-16' lengths 2" X 6" for sitting ledge sides
• 1-8' length 2" X 6" for sitting ledge ends
• Stainless steel screws for assembly

You could simplify things by removing the "shadowline" detail and the top sitting ledge if you wanted to reduce costs and just use the 5/4 X 6 and the 2 X 2 stakes.

Assemble each of the four sides on a flat area then simply screw the sections together in place.

The design of these beds allows for them to be fitted out with accessory greenhouse structures you can make from inexpensive PVC pipe and plastic sheeting to extend the growing season. It is also easy to set up floating row covers to protect tender plants from too much sun or keep pests from getting to your crops.