What's the best way to make a raised bed?

Suzanne emailed to say:
"for the raised bed- whatever is free is always good, barring that, check out places like Home Depot for their prices. I originally wanted railroad ties (very pricey) and ended up with 2X12s that I cut to length. Best dimension for garden width is about 4'- something you can reach across and not have to walk inside of, once the garden is built. So depending on the length of the hell strip, you may have something like 4ft x 8 ft by 12 in. You'll still want to dig into the existing soil as much as possible, then fill the new bed with soil (either purchased or from elsewhere in your yard), and then amend the soil."