A must for every bento collection is a compartmentalized box with removable sections. It keeps your food separate. It keeps flavors from melding. For wet dishes, it keeps the juices from running. It also allows you to reheat / refrigerate only certain parts of your meal.
Take this bento for instance. It has rice, peppered pork, tomato salad (mostly a roughly chopped salsa, really) and eggplant salad. Because we have both a refrigerator and a microwave at work, I opted to slip the whole box in the fridge, to keep the salsa chilled and for food safety reasons as well.
Come lunch time, I took the rice, eggplant and pork compartments, covered them with paper towels and reheated in the microwave. This way, only the parts of my meal that needed to be served hot was hot, while the tomatoes remained chilled. Perfect! Plus, the vegetable juices remained with them, not ruining the pork nor the rice by running. Good enough reason to get a compartment box, yes?