I would also suggest boxing the bottom of the bag. That will help it stand up. You will need some extra fabric for this step so I always cut my fabric pieces at least 1.5" inches bigger when I'm boxing the bottom. Here is a link to an easy Youtube tutorial for boxing bottoms.
https://www.youtube.com/watch?v=gJhlgKqjb0o If you are lining the bag you will also box the lining.