I also use a milk crate on the rear rack which is permanently attached there with bungee cords. As for bike, I have a 1985 Miyata ninety.
I recently got my hands on a trailer for $20 and I built an outer shell for it out of oil cloth so I could haul more groceries. I also upgraded my cassette to fit smaller gears because of the additional weight and a huge hill just on the way to the supermarket.