This is what I use for keeping multiple stacks from toppling over on me when I am stacking or burning. The red circles indicate scrap pieces of plywood, 2x6,8,10, or slabwood that connect two rows together. The scrap pieces are roughly 36" long and stabilize the stack greatly (tallest point in this stack is 7'-8'. I find the scrap plywood works best and is free. I cut it about 6" wide. I randomly place these throughout the stacks (this one is about 7 rows deep), and I've never had either end fall over during its 18month drying cycle. One of the ones in the first pic is a bit high to make a difference. Usually you want to have at least 2' of stack on top of the 'sticker' to have enough weight to hold the rows together.
Also, note the far left 2x10 that sticks out a few inches. It makes a nice beverage holder that is at the right height to keep the sawdust (mostly) out of your bee...uhmmm, beverage while working on the wood pile.