Do you take reusable shopping bags with you when you do your grocery shopping? The first step is to start keeping the bags in the trunk of your car. Eventually, even if you forget that they're there for the first few times, you'll gradually get into the habit of bringing them into the store with you. Some grocers will give their shoppers a discount for bringing in their own bags, but even if they don't, you'll probably get the warm and fuzzy feeling of self-satisfaction from helping the environment.
Once you're in the habit of bringing in canvas bags, it shouldn't be much of a stretch to add a cooler bag to your collection, which will keep frozen foods, fresh produce and refrigerated products protected from the heat of the car - an especially useful feature for a hot summer day or any time that you have a long commute. This particular insulated bag is helpfully labeled, and has sturdy wooden handles that make it easy to carry.