You need to have reusable grocery bags if you intend to use them. You can often buy these at your grocery store, but don't limit yourself to just those bags. If you look in your closets, odds are you've accumulated a number of cloth/canvas bags from conferences/trade shows, school etc. If you have these they are free and probably not getting much other use. Also, having an old backpack as one of your bags can be useful since it lets you carry more groceries without needing to bring the cart with you into the parking lot.

You can only use the bags if you have them with you at the store. This means you need some way of making sure they get back into your car. Unless you are unusually motivated, you probably will not remember /have the energy to take the bags back out to the car as soon as you finish unloading groceries (you will probably be busy trying to eat your groceries).

To overcome this, have a spot where you put the bags inside your house/apartment. It should be near the kitchen and on the way out to where you park your car. This way you have somewhere to put the bags when you trip over them in the kitchen, and you will see them every time you go to your car. The more obvious the bags are when you look at them, the more likely you will notice and remember them. A coat hook next to your main door is a good spot to start. Gotchas:If you live with other people, work out with them where a good spot to keep the bags is. Some people may object to them being an eyesore (but not take it as motivation to take them out to the car)

Whenever you go out to your car and notice the bags, take them with you and put them in your car. This way they will be in the car when you arrive at the store sometime in the future. Gotchas: