I keep at least 3 reusable grocery bags in the car; however, it took time and repeated missteps to change my behavior. If I realized I forgot my bags while already in the store or even at even the checkout line, I forced myself to walk back out to my car to get the bags. Doing this several times made it easier for me to remember to bring the bags from the car the next time until it became a habit.  After I unload my groceries, I put the bags next to my car keys so I won't forget to put the bags back in the car. 

If I am with someone else and we don't have a bag in the car, I carry the groceries in my hand or take the shopping cart to the car and unload the items in the trunk.