I use a bungee, cargo net for smaller, looser items that I want access to or want out on top for some reason: My damp clothing, my solar charged lights, maybe some food I picked up along the way. For items that are secured as part of my normal load, I have a whole strap collection: Surly Junk straps (also the loop version), Salsa Anything cage straps. Salsa's extra long straps, Nite Ize adjustable straps, and usually there's some sort of cord stowed somewhere if necessary. I could see where a cargo net could help reign in the straps on your backpack, but I can also see where, depending on how full you pack was, a normal cargo net might not be sufficient. I would likely run one strap around the backpack to keep the straps under control, and then fasten it to the rack with another strap.

Also, in a pinch, a belt will do the trick. Just remember that you've re-purposed your belt, or the next time you hop off the bike, you may have to make a frantic grab for your shorts as they head for the ground. Or so I've heard.