I have been pretty good about converting to reusable shopping bags. I love my canvas ones that I take to the grocery store. In addition to being friendlier to the environment and reducing waste, they hold more groceries, don't fall over, and allow me to take everything into the house in one trip without pinching my fingers.
However, occasionally it happens that I didn't plan ahead or remember. In that case, I always save the plastic and tuck them into little areas where I could give them a few extra uses.
Here is how I use my car's ashtray – as a storage area for these bags. Then its easy to collect car trash or recyclables, or loose items that need to go into the office or house.