Most bag patterns tell you to put a piece of cardboard in the bottom of a bag to help retain the bag's shape. I don't like this because it means you can never wash the bag, nor get caught in the rain with it.
Instead I use a piece of plastic canvas. You can even stitch it into place by machine (without any special needle) to it so it does not "travel" around the bag.
Plastic canvas is about 25 cents for a large sheet about the size of a standard sheet of computer paper, and can be found in craft stores near other needlework supplies.