I wrote an article on making biochar in burn piles that was published in Firebrand newsletter of the Rogue Valley Fire Prevention Cooperative. This newsletter was distributed as a newspaper insert to households in Southern Oregon this winter.
One reason that the Fire Prevention Coop liked my article is because of the requirement for water to quench fires. The local firefighters are always having to respond to out of control burn piles because people don't bother to put them out completely.
Since I wrote that article, I have discovered another easy way to make biochar from brush piles. This works best with small diameter stuff less than 3 inches. Start a small fire next to your brushpile. Slowly feed it with brush so you start to build up a pile of glowing coals. You do this in layers so that once the top of the pile is hot and starting to ash (the glowing coals turn white on top), you add another layer of brush. Wait for that to heat up and then add another, until all your material is used. You should have a nice big pile of red hot coals. Now put it out with water. You could also smother it with dirt if there is no water nearby, but you must put it completely out or the coals will continue to burn and you will not get biochar.
Another advantage of this method is that it burns much cleaner so you will get less smoke, and fewer complaints from the neighbors.
Make sure you compost your biochar (try manure, worm castings, kitchen scraps or bokashi) before you add it to the garden. You can also soak it in pee before composting for more nitrogen - it really sweetens up those chamber pots!