Now I can honestly say I have done this one and its really enlightening.
I first recommend that you stop throwing food scraps in your garbage if you havnt already done so. Its a minor adjustment if you are already composting your scraps. You will need to come up with something to do with your meat & bone scraps for the week. Ask your kids to bring home their lunch garbage from school for the week and you bring yours home from work.
At the end of the week, just before garbage day spread out a big tarp and gather the whole family together. Make sure its not windy and make sure everyone has gloves on because its going to get a little messy. Once you have opened all the bags from the week and spread out all of your garbage all over the tarp, take a moment to look at how much garbage you have then start sorting. There is no lesson more profound that seeing how much and of what you throw out every week.
Identify what could have been recycled. Here is a golden opportunity for redemption (value). Look at what you are using the most of. Are there alternatives that create less waist? Treat it like a science project and take notes on what you find.
After you have cleaned very thing up and have it re-bagged for the garbage truck, sit down and talk as a family about what you found out about yourselves and your impact on the environment and how that differs from you beginning assumptions about your own habits.
This is a valuable first step in reducing consumption and waist. Now you are ready for the discounted 20 gallon garbage service. See if you can beat us! We come in around 6-10 gallons of garbage per week.