I try to ask customers if they "need" plastic bags. I do this purposely and avoid the word "want." Sometimes you just know someone wouldn't think of not taking a bag, so you sack up their purchase without that conversation.
Occasionally you will get a customer that asks for a bag and then is really excited by the bag. "This is a nice bag!" they say, "I will definitely reuse it!" These people are the ones that have been trying not to take bags when they do their shopping, only to find that they rely on them at home. The reused bag is the doggie poop bag, their trash liner, their car seat cover. Those plastic bags which were once a nuisance and crowded their cupboards are now precious objects and guilty pleasures. They take a bag because they need one to be a shield between their hand and gross realities.
I will admit that I had that thought. "What will I line the trash can with?" But you know what I did? I just didn't line it. And though the trash can make get a little dirtier without its plastic bag condom, IT IS A TRASH CAN. It is made for garbage. I hose it out once in awhile and call it good.
Try it customers! Set yourself free!