Green bag promotions lineup

A friend of mine at work goes to Subway Sandwiches almost every day for lunch.  This week he came back with a cute little fold-up zippered tote bag that they sold for 99-cents to encourage re-use.  Apparently the promotion is that every time you bring in your re-usable green bag and buy a sandwich, they'll give you a free cookie--limited time offer.

I walk to Trader Joe's a few blocks from work about once a week.  It is a struggle to remember to bring my own bag.  Since I'm not in the car, I don't have the luxury of simply diving into the trunk for a spare bag.  Any ideas how to remind myself?