I plan by menu - when that is done I can pack accordingly. When tripping I get rid of as much of the pre-packaging as I can and will re-package to accommodate my needs. I have a colleague who is into dehydrated foods and does much of his own especially ground beef.
We are fortunate to have a lot of choices we can select from - knowing were to find them, bypassing the Outdoor Supply Stores, can be tricky. But it can be done at great saving.
The secret is planning a menu - and not leaving it to the last minute like I've seen a lot of people do.