If I forget my reusable bags, I don't buy more, I just ask for paper bags (usually), or occasionally plastic if I'm ready for a six-month supply of wastebasket liners. (Most of our trash receptacles are much too large to be fitted with grocery-size plastic bags, but a few small ones accommodate those little bags.)

So I still accumulate collections of plastic and brown-paper bags; I just don't collect as many as before, and not nearly so quickly.

Of course, the bags we use to transport groceries from store to car (or bus) to home are not the only instances of wasteful packaging, but it's something we can do something about. And it's not all that difficult or expensive. Just don't worry about it if you occasionally fail to comply!