Some of my properties will allow me to squeeze a small, light trailer in places right at the work, where I could never get a chip truck anywhere close. The uphill pulls are significant, too.
To slash down most effectively, each round, never step on things being cut. Binds up the saw, robbing power. I'd start cutting outside the box for my first slash through, about a foot from the tailgate, from the sides or rear. If climbing in and working from the rear, keep the saw tension right, and WOT. You can lay the powerhead on the brush in the middle of the truck, and have the tip at the bed rail, allowing you to have tons of control while starting, and burying the bar, working your way across, keeping your tip down, or flip over and cut toward the middle.
That's putting the cart before the horse...stacking for success is at the start.
Smaller and straighter in the bottom works best.
Wait to load bigger pieces (as possibly, without 'swiss clock'-ing it) until the final slashdown. If you have big pieces that end up at weird angles, its lots more work to slash through, again and again.
Tarp.
Odd pieces/ long pieces on top.
Be good at holding your breath, then taking a break to breathe fresh air, or have a leaf-blower on blowing on you.