I've done this two ways, one worked a whole lot better than the other.
The first was when we cleared about an acre of land for the house. It was 99% brush, one or two good sized trees, and about 20 very large, old stumps. The stumps were dug out with a rental excavator. We piled the brush and then the stumps. The slash pile was enormous. Diesel, a brush fan, and propane weed burner got it going and it burned pretty well for about 2 days. I was still left with about 8 still large stumps. As I cleared more brush, I collected it in another large pile, rented an excavator again, cut the stumps down as much as I could with a chainsaw, and burned again. After that I was down to about 5 large sections of stump, these were buried, as deep as the excavator would reach, in a spot that will never be utilized as anything but pasture. 4 years later I still haven't seen the ground sink in that area.
Here's a picture taken of one of the larger ones during the restack (barely touched by the original fires) I'm to the left a friend is running the machine. For the what the rental of one of those costs and how much fun they are it was hard to give up the seat:
A few years later I hired someone to clear my barn pad. There was about 40 trees to pull out, half of them over 12" at the base. The guy used an excavator to topple them and limb them. I cut the root ball off and bucked them while he held them up. He piled all the limbs and stumps in a very particular order. When he was done I had a pile of limbs and stumps that was about 15x15x15. This was in the spring, I figured I'd have to let it sit till fall before burning it. He told me to light it now, and he'd be back in the morning to restack what was left. I figured he was out of his mind, these trees were alive that morning and were still plenty wet. The weed burner and leaf blower trick got it going, I was amazed when the whole pile took off. That pile was burned to nothing in 2 days. He said it's all in how you stack them.....amazing.