I burn a lot of brush piles and what I do in the summer is bushhog aroud the pile a few days ahead of time and burn at night, heavy dew on wet grass is not very flammable. There is not a bruch fire yet that I haven't had get out of hand, I'll usually use a bale of straw for ignition (we have a bunch of it, cheap commodity here) and if its doesn't go them i'll add a mix of deisel fuel and oil, or just straight used mtor oil, or just deisel fuel but of course whenever it extenguishes itself. In late winter early spring if I have a big pile to burn I have been known to take a rather large disc around a field to create a good fire break and just burn the pile and field off at the same time. I prefer this way because it's easier to control, disc the downwind side and the edges real good, light the downwind side first and you have created a fire break, then light the upwind side 5-10 feet from where you disc'd it and monitor the cold burn areas to make sure they don't creep across your dirt fire breaks. Usually I have a buddy or two with me in the field and we are all on four wheelers which makes it easier to monitor/extenguish/light. let the field burn and the brushpile will probably catch fire and go, if not add deisel fuel and light her up.