As Erik mentioned, you'll want to use a surfactant. With the brush killer, the most common surfactant is diesel. Read the directions but I believe it has to due with it mixing properly.
For burning, I found the sole use of the rosebud torches that Erik mentioned too slow when you need to burn in volume (and uses a lot of propane). What I do now is to take a 2 gallon pump sprayer and fill it with diesel (or kero). Add 1/2 to 1 quart of oil and shake/mix it up. Spray that on the bush and then torch it (using the rosebud or anything else that's good a getting diesel burning). Since the bush is now covered in fuel, it burns on its own.