Gary:
I have experience as both the brush burner and the firefighter who responds when the pile gets out of control. Here are a couple of ideas that have worked well for me in the past.
1. Big piles do not burn efficently. While it is more work, having a hot smaller fire, 10' x 10' and feeding the wood into it, consumes it quickly. It is difficult to get the very bottom of a large pile to completely burn. Establishing a hot bed of coals, and feeding the wood on top, easily burns even green and wet stuff. The more complete the burn, the less smoke generated, and the less complaints from the neighbors.
2. If possible, start the burn in a pit, dug by your tractor etc. This contains the fire area, allows for "radiation feedback" and easies coverup and regrading after the fire.
3. While it may seem obvious, check the weather report for the wind conditions. The larger the pile, the more embers and brands generated. Have a friend assist in patroling the area, to extinguish any spot fires that might develop.
4. Use caution in starting a fire with flammable liquids. If a small amount gets the fire started, using alot only makes it more dangerous for you.
A safe method is to have a metal container at the base of the pile, with 2-3 gallons of diesel fuel. Run a trailer line of diesel fuel to the outside of the pile, and ignite with a road flare.
Motor oil generates a large amount of black smoke. Gasoline vapors are heavier than air, and will find a remote ignition source, perhaps lighting you on fire.
5. Where I live in CT., a permit is required from the local Fire Marshall or burning official. They will inspect the area for hazards and establish the conditions to allow you to conduct your burn. Having your burn get out of control, and spreading to your buildings, neighbors, woods, etc. uses up your local Fire Depts. resources needlessly, when the origin is most preventable.
I'll get down from my soapbox now, but reiterate that the burning the smaller hotter pile takes more work, but will give you a better result. Also, if you invite some fireman buddies over to assist you, get most of the work done, prior to taking the lock off the beer cooler.
Good Luck with your fire:
WALT "The ol' Captain"