It's a simple concept, really. We just can't seem to master it.
Unlike grocery stores in the States, stores here in France charge you for the bags you use to carry your food home. There are varying qualities of bags at various prices -- from flimsy plastic bags to strong, sturdy canvass bags (the shopping carts even have nice little places to hold the bags while you shop). The French will use the same bags over and over for months, making sure to always take them with them during trips to the market. Our kitchen cabinet is full of dozens of these sorts of bags, ready for reuse the next time we make a trip to the local Intermarche, Champion, or Carrefour.
But it never seems to work out that way. Instead, we find ourselves in line to check out with no bags, requiring us to purchase a few more to add to our growing collection. Forgetting our bags isn't such a big deal during those spur-of-the-moment trips to the store for a quick baguette or some milk. But when we leave the house for only one reason -- to go shopping for food -- and we still forget to throw a few bags into the car it gets annoying...really annoying.
Perhaps there is a trick we haven't learned or a way to remind ourselves to bring bags with us when we shop. But until we master this BYOB concept, we're going to have a surplus of really nice grocery bags filling up our kitchen cabinet.