I discovered these bags online and fell in love with them. They are designed to stay open so you can throw trash and wrappers away while you're in the car. There is an outer shell with a decorative fabric and an inner-shell made of a stiff fabric. You can use a very small trash bag as a liner, or if you're like me, a Wal-mart bag. (I always seem to have those in abundance.)
The bag's strap is expandable and secured with a safety pin, so it can be used in different areas of the car and adjusted to your liking. Here are some ideas of what they look like and where to put them:
Each bag costs $14.