My local grocery stores all "encourage" the use of resuable bags.  They sell them right at the register, and occasionally will even verbally thank you for bringing your bags.  Well, envrionment aside, this is clearly in their own interest--they save money on the plastic bags, and I feel sure they're making a profit on the bags that they sell you if you buy theirs.  At the same time, there is a cost to the customer--you must buy the bags, and you must remember to bring them each time. 

I propose they share some of that incentive with the customer, too.  Even something as simple as special coupons reserved for bag-bringers.  Make the coupon for something else environmentally friendly (natural cleaning products, organic veggies) and I think you have a real motivator that could be sponsored by relevant companies!

Yes, the environment should be reason alone to do it, and for many it is, but something extra might convince that next tier of shoppers who are on the fence.