This:
Detours Toocan bike panniers. They cost $60 each. they are almost top of the range for your regular mom-on-a-bike. I got a rack strong enough to take them that cost $40, and one of these. Then another. They are indestructible. I'm you're regular cheapskate walmart shopper, but the wat i load up my bike, I've killed several racks and panniers. These rock. Right now my luggage system cost half as much as the bike. But i can load it up as unevenly as I want ans still cruise no hands. Awesome.