The biggest eye opener for me was when we started seriously composting in my house. Food scraps, paper scraps, chicken bones, etc etc. It pretty much halved the amount of garbage we were putting on the curb. Ironically, a normal weeks worth of garbage now fit's in a plastic grocery store bag.
I have two resusable bags that I aquired in 2007 and I still have them. They are made from water bottles, not sure how it changes the 1 to 20,000 factor. By my math this has helped me avoid using about 2,000 plastic bags.
12 years
1 shopping trip a week
75% usage of reusuable bags
2 plastic bags per one resusable (probably the low end, I bet its more like 3-4 plastic bags in one load of my reusable)
2 bags
12*52*.75*2*2=1872