Trader Joe's grocery bags
I like the reusable grocery bags from Trader Joe's. They are just the right size for my drybox, stand up on their own, and they only cost a couple of bucks each. I generally use them to hold food organized by snack or meal, so when it's time to be used, you just pull that bag out and carry it to camp. It's the same concept as the Mountainsmith bags, which I've coveted as a total gear geek, but a lot cheaper. Downside to the TJ's bags is they can't be zipped closed like the Mountainsmiths, so if you do go wrong side up they won't hold everything in.