I picked this up from Wikipedia. I hope it's okay otherwise someone please inform me if I'm contravening some kind of law or something.
A bento is traditionally made in a 4:3:2:1 ratio: 4 parts of rice, 3 parts of the side dish (either meat or fish), 2 parts of vegetables, and 1 part of a serving of pickled vegetables or a dessert. However, almost anything can be used to make a bento.
Several precautions should be taken when making a bento. The most important thing is to avoid food poisoning, especially in summer. Foods should be cooked well and the bento must be stored in a cool, dry location. If sushi is a part of the bento, it should be prepared with more wasabi than normal. Foods covered with sauce should be carefully packed, or avoided altogether, as sauce may spill over on to the other foods. When cooked rice is a part of a bento, it should be cooled by leaving the bento open first. If a bento is closed before it is sufficiently cooled, the steam from the rice will condense inside the bento and make the food soggy.