Restaurants tend to hilariously overestimate the size of a human stomach.
All the arguments for bringing reusable bags to the grocery store apply to restaurants as well. When you inevitably can’t (or shouldn’t) finish your food, put it in the box you brought with you. As a bonus, you won’t have to wait to catch the waiter’s attention and then wait some more while they go fetch a disposable box for you.
Of course, prepared food tends to be a bit messier than groceries, so you’ll probably want something with a watertight lid. Containers with more dubious lids can be put in a plastic bag to catch potential spills.
Another difference from grocery shopping is that restaurant dining is often not premeditated. I tend to end up at restaurants having forgotten to pack an empty box when I left home that morning. One way to handle this would be to keep a box in your purse / backpack / car at all times, but I’m still on the lookout for other suggestions. If you have ideas, please comment below!