[vulcan_dude]

I place the backpack shoulder straps against the sissy bar (sandwiched between the sissy bar and the backpack), this keeps the straps from moving, and the backpack sits on the rear seat, facing forward.

Then I attach two good quality bungee cords (not the cheap kind that stretch easily) to the sissy bar and wrap them around the front of the backpack, one near the top, and one near the bottom of the backpack.

The quality bungee cords keep a very tight hold on the bag (this is why it's important to NOT get bungee cords that stretch easily; a lot of stretch will have a lot of give and the bag will move).

I've traveled several hours at a time with this setup, and short trips to sites with no problems whatsoever; turns, curves and quick stops are no issue.

The key is to ensure that the bungees you get have a lot of resistance, so that when you stretch them around the backpack, it is tough to move the backpack with your hands.

Nets are cool and work great, but I wouldn't use them to keep something heavy like a backpack in place, their stretch resistance is weak - they stretch a lot, and heavy things will have no problem shifting in the net. I use a net to keep stuff like extra layers of clothes/chaps/jackets/extra helmet in place.